Kiwi,
It is never a bad idea to slow down when you are working on a bike like this, especially if it is one you don't have a lot of prior experience with.
Any 1962 Model 88 is a very rare machine, especially one that is as complete as yours is. The parts specific to your engine, the cylinder and crankshaft especially are extremely rare, if you are lucky you will see one of each come up for sale on the world market in useable condition each year, really.
The rest of the engine parts are easier as they are the same as either the Model 99 or 650. In fact your crankcases are what some people call "humpback" Dominator cases, which they just started using on the 1961 bikes, it has more clearance in it so that it can take the big 650cc crank and connecting rods. In fact all you have to do to convert your engine to 650cc is to put a 650cc crank, rods, pistons and cylinder on it. Your cylinder head is not the style the 650 was sold with but it will bolt onto a 650 and run fine.
If you can get a first-rate machinist with experience sleeving bores on classic bikes and a list of satisfied customers then it would not be a bad way to go. I am running a 650ss right now that has a sleeved cylinder and it seems to have no problems. A good machinist with experience would be able to look at your cylinders and tell you how much material has to be left in them before they can no longer be sleeved. The 88 cylinders do have thicker castings than the 99 and 650 cylinder though, and if you run a good air cleaner the cylinder can last a very long time. I know someone who has an original paint 1961 Norton 650 that has tens of thousands of miles on it and still runs well with it's original bore and rings because since it turned past 5000 miles it has had foam sock filters on it's carbs.
The current Hepolite company is supposed to be selling all sizes of Dominator pistons, and a very good Norton engine builder here says that he has had no problem with the quality of current Hepolite products. The Norton 88 has small light pistons compared to the 750-850 Nortons and it has a very short stroke, the pistons are not stressed highly, so any cast pistons you can find will do a good job, leave the forged pistons to the racers.
My first choice for Norton engine parts is to be patient and wait for old 60s stock to come up for sale at flea markets, old dealers or one of the Ebay online auctions.
Norvil has a large selection of parts, but a lot of his parts are sourced from third-world suppliers or are not manufactured to original specification. That is okay for a seat or something less critical, but not for engine parts. Most of Norvil's positive reviews they generate themselves. Gaskets are not really critical parts so I might trust them to supply those.
Andover Norton mostly sells Commando parts, but their parts are all made to high standards and must be to original quality spec or they will not sell them. They have a few parts for Dominators. The 750 Commando guides are the same as Dominator. Commando valves will fit but are a bit taller so they either have to be ground shorter and re-hardened or your pushrods need to be shortened to work with them. Norton made the longer valves an across the board change to all their Dominators starting in 1968.
Standard Commando valve springs are the same as SS Dominator springs and will work in your engine just fine. They are a different free length, but that is no concern as long as they fit your retainers and cups etc..
If your bike is something you are planning on keeping for life, then take your time and make sure you do a good job on the engine and gearbox at least. I am not a fan of bead blasting the alloy parts of vintage engines, it ruins the original as-cast finish, but one place bead blasting is very helpful is in removing the hard carbon deposits from around the valve guides in the head ports. After this carbon is blasted off, then heat the head up to 150c-200c and with a nice fitting drift knock the old guides out and if they are handy the new ones into place. If the old hard carbon is not gone and/or the head is not heated up when this is done then you will score the guide bores and ruin their ability to seal against the new valve guides.
The less you do to these old engines the better. When I looked close at my 62' Domi engine I saw that the cylnder had been sleeved to standard at sometime in the past, it was used though. I measured the bore and found it was worn .0035 over standard with no noticeable taper, the factory said that up to .006 wear with no taper was good so I threw new compression rings in it, gapped them and was finished. I put NOS iron guides in it with some new valves I had bought years ago, made sure my used springs were not sagged according to Paul Dunstall's tuning manual. If you want me to email you a pdf copy of his manual shoot me your regular email address and I may be able to do it, it is less than 8mb file, lots of good Domi information in it even for stock rebuilds.
I have my own valve grinding equipment so I kissed the seats of my head as lightly as I could to get the job done and threw the head together.
Superblend main bearings that they put in 850 Commandos will go right into a Domi and they never go bad. I have a few used sets I saved out of Commandos I had scrapped out years ago and put a set in. Often you can get these from Norton engine rebuilders that they have taken out of engines that customers bought new ones for that are in excellent condition very cheaply.
My crank rod journals looked very good with a micrometer so I cleaned the crank out, made sure there were no burrs inside it where they drilled the oil holes through and put it together with new std. rod shells. Before I had weighed all my reciprocating parts on a gram scale and balanced the crank on my kitchen table on two knife-edge levels.
My Domi runs very smoothly and it is very powerful now. I put a plastic-caged drive-side layshaft roller bearing in the transmission as this is the one upgrade for these gearboxes that is so good it has to be done. The rest of the bearings looked very good, so I loctited the sleeve gear bearing in place, made sure my layshaft end-play was in spec and put it together, it shifts like slicing warm butter. A mouse could shift it from 3rd to high for me.
Don't replace things just for the sake of replacing them, just the critical or safety items. If something is good use it, you may not be able to find modern replacements that are as good a quality.
The 650-750 both had the same cranks, just balanced differently and are very easy to find. All 650-850 rods are interchangeable. 650 cylinders are hard to find but not as hard to find as 88 cylinders. 99 cylinders will work with a 650 crank as long as the bottoms of the cylinder sleeves are notched to clear the 650 crank and rods. So if you run across a good 650 or 99 cylinder for a decent price grab it, as you could run your engine as a 650 for a while, as long as you don't mind having extra torque and power and reliability.
Oiling system: Very marginal on these bikes. The 650ss, 750 and all 64' and later Norton twins had higher capacity pumps, 66' and later doubled the speed of the pump with a new set of drive gears. A later pump and drive gears will bolt right onto your engine. You will have to carefully enlarge the oil passages to fit the larger passages on the pump body. Increasing oiling is not for racing, but for reliability and so you can push return oil through a Commando style oil filter, which will make your rod journals last way longer if you put high miles on the bike. The oil tank with the froth tower has a fitting that you run the engine breather to so oil consumption is improved quite a bit. Ex-Norton employee Heinz Kegler told me to leave the breather disc off the end of the cam and to drill a new hole in the crankcase where I did. I added the oil tank with the froth tower and put the old one on a shelf in the basement. Seems to work well. The later Dominator oil tanks can often be found very cheaply. The 66' and later Dominators had plain rocker arm shafts with full pressure oiling to them. Your rockers have scrolled shafts with a low pressure feed to them off the return line to the oil tank. I left the old system on my 62' 650ss as I have seen bikes with this system go many thousands of miles with no problems. I had the parts and used them as it was easy to use them. A top Norton engine builder here in the USA did tests and found out that if he drilled a new drain hole in the timing chest to lower it's oil level to the point where the crank timing pinion is just kissing the surface of the oil level, that the engine oil temperature dropped significantly. Easy to do on any Norton with a hand drill.
Here is my 650ss going together here with details on the oiling mods:
https://www.facebook.com/media/set/?set=a.248150218565584.56645.187441501303123&type=3 So you get back to taking your good old time with the bike. There is a lot to learn and a lot of subtle things that can be done to improve it's reliability and practicality. . I was in a hurry to put my 62' together and although I did a decent job and it runs great, there are still things I would have done a little different if I had waited a while longer and taken more time on it. Hurrying was my no.1 enemy and it certainly was my bikes.
I guess that is enough to think about for tonight......
