2 stroke China Girl is definitely the way to go on a motor bicycle with these conditions,
1. You have blue print your motor before installation in the frame
2. or, you need to plan on buying about 5 motors on average it get 1 good one out of the box. Still not as bad a deal as it might seem, because you'll have spare parts to keep you going for years.
The advantage of a China Girl as I've described is, it will be faster and lighter, plus just as smooth, and dependable as a 4 stroke of equal size. Even if it's not faster it will still be a lot lighter. China Girl motors are easier to work on, and less maintenance intensive. There's a better aftermarket for China Girl parts, and performance.
When all these considerations are taken into account, the China Girl advantage is pretty much insurmountable. Here's two China Girls to prove my point, that also happen to be my own.
I must be right because my wife said something about me knowing it, and it all lol.