Not much you can do to a sidevalve engine...hardly any room for larger bore, exhaust tract ussually crap so tends to overheat lol

The only good thing is that it is easy to maintain...then again it breaks a lot so it had better be easy to work on lol

I remember that the Harley WL750 can be upgraded with beltdriven primary, needlebearings in the transmission and so on.

Best thing for it really is to take the engine apart and re-engineer some bits, upgraded things like bearings and stuff I guess