John,
Believe me, your in the right place. As a member you can use the search function now... Just ask if you need help, it's a little tricky.
20in. choppers are cool and could be a little difficult... There's a number of small obsticles to overcome. If you've got the money great if not, let me stop you and recomend a beach cruiser or Moutain bike!
There's many places that sell custom parts to make the adaptions, allowing a motor to fit. First will be mounting the motor, there's many mounting brackets for sale to make it possible!