If it's a steel frame, then it should be Ok. Avoid aluminum frames. I would suggest a pocketbike engine so it can fit inside the frame, however you'll need extra wide cranks. A rack mount would also be advisable for a small frame.
Welcome to this motoring thingy and enjoy the build as well as the ride...
The 50 and the 80 are both the same size. The reason why Joe mentioned the "70" is because there is no such thing as an "80", but nevertheless, that's what everyone "sells" them as, when in reality the 80cc is actually 69cc.