re: 60 MPH...achieveable? Who cares?!? Let blueprint an engine!
Weight matters on a drag strip but doesn't have much effect on top speed on level ground with lots of room to accelerate. Wind resistance is the real problem. The power required to move a bike at 60mph is 8 times that needed to go 30mph. That's at steady state speeds not counting acceleration from 0 mph on a dragstrip. So you'd need even more power to get to the desired 60mph due to the limited amount of time available to achieve that speed. Whether it can be done or not is up in the air. I tend to think it could be but not with a stock China girl motor kit. Besides the work on the engine that's going to be needed the lightest chain and thinnest sprockets and perfectly mounted rear sprocket is going to be important. A chain tensioner would be a real drag and should be eliminated. A streamlined fairing aft of the engine might help. One thing that might be worth considering would be turning the piston and cylinder around and replacing the intake filter with a ram horn or simply making an air ram horn that wraps around to the carburetor in the usual location. That won't give as much boost but it would still help. Small tires and lots of pressure would also be important. The important thing here is to get the most power possible out of the engine and the least drag in the chain drive system. Gluing some shrink wrap to the bike wheels rims to smoothly cover the spokes would also reduce some drag. That's what was done on WWI aircraft that had spoked wheels. It really is an interesting challenge.
Goldwing 1500, Silverwing 600, Schwinn 49