Be careful of sidevalve Jap or Sturmy archer engines on offer, as most on sale via ebay seem to have started life in a lawnmower or some industrial application; sold by people who have very little knowledge of vintage or even british bikes.
That said, a sv Jap engine ought to be the easiest to fit, and for spares.
I'm none too sure about that gearbox either, if it came with the engine, its likely lawn mower as well.
What Cardan says below is spot on but I disgree in part with what he said about people not willing to share what they know.
I quit one club [not VMCC] after years of sharing extensive knowledge of one make gained from, not only owning that marque for 50 years, but a stint working at the factory that made them.
The reason for that was after sharing such knowledge, gained the hard way, and not by reading misinformation from the old bike press invariably up would pop some born again type who tried to contradict me or prove me wrong, or show me a "better" way of solving a non existant problem. [I'm a firm believer in doing things the way the maker intended and putting the thing together, once, properly. So many think finding a hi tech solution to a low tech problem proves something].
There is only so many times I can stand being told how "wrong" my own bike is, the bike thats had two owners in its 80yr life.
I came to the conclusion that one big factor in that was the newcomers to old biking, almost always the worst kind, are the guys in their late 40s or 50s who have made a few bob being successful in life,bought an expensive bike but cannot stand to be shown to be mistaken or ignorant of a subject. Needless to say they have spent their time in non practical businesses.
The other is the type who has read all the classic bike press, thinks he thus knows it all, and is therefore an 'expert' on it all.
That the type who is selling stuff on ebay so hilariously misdescribed.
As for the bike press, I stopped reading many years ago when the 'expert' writer did not know the difference between a magneto and an alternator. A lot of the time these writers are just re-hashing old misinformation from older issues.
That shows the value of the older resources, now being cut up and dispersed by opportunists on ebay etc.
You cannot beat first hand knowledge.
As for you smartfella ask around as much as you can, but wiegh up the info you get, judge it, sit on it, and then do not be afraid to dismiss it if it doesn't feel right.
[and don't waste money on white wall tyres]!
