Re: 50cc or 80cc
I have one of both, and even though it may just be my particular motors, I can see a HUGE difference. Basically to the point that the 50cc isn't really worth the time. Again, i may just have a slow 50cc motor, but it accelerated much slower than my 67cc and didn't reach anywhere near the top speed.
You would probably be happier with a 67cc motor.