Hey Paul it's chris! I didn't know you were on this forum
I think some people sell those power trailers already built and ready to hook onto the seat post of any bike.
I think to add a 5fth wheel to a 4 wheel bike would just be a matter of being able to lift the wheel off the ground or be detachable like those trailers. Or if you go with a friction drive you could have a fifth wheel with a friction drive that can pivot off of the wheel. So when you want to pedal you can quickly pivot the motor above the wheel. Or vice versa.
I can't find the video where he shows how easily it pivots up and down when you want using the bicycle style quick release clamp.
You don't need a particularly powerful engine if you're going around the neighborhood. Any 50cc should push around a neighborhood and some hills at 20-30mph depending on how you're geared. Most engines you buy have built in centrifugal clutches and just need gearboxes. Which thatsdax.com has (youtube video is of duane at thatsdax)
I hope this helps in someway