Re: Propane conversion
I tried this on my lawnmower awhile ago. The carb was clogged up and I didn't want to tear it apart to clean out the jet again. So I found about 1 foot of vinyl tubing and put one end on a blowtorch, and the other into a hole I drilled into the intake box. It started right up on the first pull and ran for about 15 minutes at a time before the propane bottle iced up and the flow petered out.
This was a roughly 100cc engine, so I imagine a 50cc bike engine would run better without icing up the bottle. Although one go-kart mechanic I talked to said that propane burns hotter and can overheat your engine, so watch out.