Hi Technocyclist, here are some pics. of a pocket bike motor, this one has the "big bore kit", there are mounting holes on the bottom. There are some pocket bike transmissions available, the one pictured has the clutch bell and 3.25:1 ratio so u would have to go 6:1 on the final drive to get a 20:1 total.(probably hard to find a small enough drive sprocket. Besides the one pictured is a oddball 8mm sprocket.
Like u suggested a jackshaft would probably work better , there are also a CVT (continuously variable transmission)transmission available.
I used a one off rear wheel from a Honda moped with a build in 12:1 gear reduction with a neutral lever.(http://motorbicycling.com/f36/work-progress-3733.html
) I used a Stanton clutch on my build.
Rack mounted would work best as this motor combo is quite wide.