Re: 70 degrees or 75
66cc is best for power and overall practicality. Pretty much any bike frame with disc brakes, a suspension fork and a big enough spot in the frame.
You can mount the engine in literally any orientation. There's a member who mounted one upside down. It's a two stroke, they're not as pesky as a four.
Basically, get a bike, fit the engine somehow, and make it work. The engine will run fine.