Have you considered valve clearances? Could be behind noise & starting issues.
Excerpt from a competitor scooter forum: "as far as symptoms go, it would be performance related and or engine noise...if your valve clearance is too wide you will get a lot of what sounds like ticking or clicking. If the clearance got soo wide, you would lose a great amount of power because the engine would have a "bottleneck" and not be able to move enough air. The specific reason for this is with a large gap the valve isn't being depressed as far as it should and not allowing the proper air flow. Likewise, if your valve clearance is non-existent or unnaturally on the negative side you would lose performance because your valves would not close all the way and not provide that all important seal it needs during the various times of the stroke cycle. The sound that this symptom could make depends on the valve out of adjustment, anything from backfiring to blowback. In either case no engine damage will likely occur and the scooter will be unrideable before something could break..Hope this helps"
