Re: 20 inch?
1) Depends a great deal on make of bike, if you're talking in-frame. (I've seen pictures, so I know it's been done.)
2) You sure could put a rear-mount engine on one.
3) in-wheel electric motor is probably also an option.
"There is nothing wrong with wanting a motorbike that is an extension of your personal taste and fashion sense; if you must ride somewhere, I say do it with style