You're kind of a big guy to go on a c70 in any case, aren't you? I'm 5'10 / 180 and while I grant my little 50cc Express isn't a lot of power under anybody's hiney, 72cc wouldn't be that much more under mine.
Considering everything you want to do, you might consider buying a stripped frame and buildling a bike from the frame up, from scratch.
Take a look at this:
http://www.dieselbike.net/video/video.html
You're not the first guy with this idea, also Google Changzuki for a guy who started with a perfectly functional bike, pulled the motor so he could stuff in a Chinese diesel.
I'd like to see one with a small Caterpillar C.7 engine. In Industrial Yellow, of course.