What you're attempting is very complicated (& not for beginners)... I hope you have some serious skill at fabricating, because there isn't much out there that's 'off the shelf'.
What I suggest is looking at Sportsman Flyers pedal assembly... http://sportsmanflyer.com/shop/parts/sportsman-pedal-crank-assembly-with-reduction-hub/ ... this might work for you!
While you're there, look closely at all his motor mounts & maybe ask Pat (the owner) if he has any spare mounts laying around that might work for you
Good luck & post lots of pictures of your progress!