So I ended up going with a 4-stroke friction drive and the engine works amazing. It's a 49cc engine. To my surprise, it's not a total eyesore like I expected. It looks like it belongs. :-D I only had a little bit of time to play with it today. Mostly letting it run on idle for about 30 minutes with a bit of playing with the throttle. I only played with it on my back porch but I noticed something curious. When I got on the bike and carefully eased the throttle, it made a terrible high pitched squeal and moved quite slowly despite high RPM's. This was trying to move from a standstill though. I sadly ran out of time to take it to the road to test this theory out but... Are these engines meant to be engaged at speed? It sounded like the clutch was slipping from a standstill.