Re: Engine won't go past 30MPH
A 50 mph china girl is going to have a very short lifespan. You need about 5 1/2 to 6 hp to do 50 mph.
After careful tuning and break in, most of my china girls would top out about 35 to 40 mph with a 36T on 26" wheels. They cruise at 30 mph pretty dependably, but riding at 35 vibration takes a toll in short time. 40 mph is a lot worse.
It's not a good idea to turn a bicycle into a motorcycle.