Any modern oil made for manual transmissions will be fine. If I remember rightly Castrol call theirs VX90. Probably even if in perfect condition you will find your box leaks to a certain extent as it predates modern oil seals. The odd felt ring around various shafts is the best they did back then. Also, modern oils will creep through where the old oils couldn't. This is a good thing as you know oil is getting everywhere although it can make things a bit messy at times. Penrite make a semi fluid oil/grease for this type of application. It is slow to flow when cold and becomes fluid when it warms up in use. Many owners find this solves the leakage problem nicely. The choice is yours, all are light years better than any 1926 recommendation from the factory.
As far as spark plugs go, remember your bike is not a modern high compression, highly tuned sports machine where the wrong grade of plug will have nasty consequences but a low compression woolly sidevalve built at a time when designers were only beginning to understand about heat ranges. It doesn't matter what grade you put in, it won't hurt the engine. At the extreme ends of the heat range you may find either the plug burns out sooner than you like or possibly oils up but thats about it but any middling plug will work fine. For that matter the 18mm range available nowadays isn't that great and is mainly made for older engines plus the heat range a modern plug spreads over is far greater than anything available when the bike was new. You'll find something like an NGK A6 or equivalent will be fine regardless of how you ride or what you do.
Vintage bikes are fun but they do require a different approach from modern bikes. But thats why you bought it isn't it.
Cheers,
