You'd be better asking these questions on endless-sphere . Nearly anything can be done if throw enough money at the project. Simplest would be a $300, 1000 watt hub kit on the front, but of course that's not what you asked. If you are limited to a small rear axle sprocket you will need at least a single level of gear reduction on the motor before the final drive or it will be easily bogged down. The 'Stoke Monkey' system you describe uses the gear reduction of the drive train as it assists the left side crank and by shifting gears you keep the motor in an efficient range.