Welcome walker to the forum, glad you could join us
I have a 80cc (67.5cc) and I've been very happy with it. Some guy's say the 50cc rev's faster and is faster then the 70cc. But the 70cc seem's to get you up hill's a little better. I think getting a kit from a dealer is better then e-bay, seems to be alot of missing parts and trouble getting help from e-bay kit's. ThatsDax has been very helpful to the guy's in the forum and alway's takes care of any problems right away.
Here is a link to what bike to get a hope it is helpful. Any bike can be used especialy if you can weld. But the diamond frame bike seems to need little or no mod's to get the motor to fit.