We made a starter motor powered trike (no pedals) several years ago. It was recumbent-ish delta format with drive to one wheel only. We used a Mini starter motor with the 'throw-out' inertia bit removed. You need leads that will take the 30-40ish amps and keep the leads short. we used a starter solenoid with an on\off push switch on the handlebars.
We tried a couple of diferent drive methods:
First off, we used a 9 tooth sprocket on the motor, and (i think) a 42 tooth BMX chainring as a sprocket, filed out and welded onto the normal sprocket of a 3 speed Sturmey Archer hub in a 20" wheel. - I think we ended up pringle-ing the wheel with the cornering forces and I know we split the 9 tooth motor sprocket.
I think the welds were going on the chainring/sprocket joint as well.
Secondly, we used a different 9 tooth motor sprocket (from a lawn mower) with a chain running to a sturmey archer hub with the biggest sprocket Sturmey used to do - probably 28 tooth. The Sturmey hub had been taken out of a wheel, and emery cloth glued round the body. The hub then drove the rear wheel by friction drive on the tyre. it worked better than we imagined.
The starter motor could only be run for a few minutes at a time as they have no cooling and we burnt our first one out. We were getting speeds of about 20mph from each setup.
You could use a dynamo off the old Mini or Morris 1000 - Unlike the more modern alternators, they could be run as a motor and had a fan and, I think bearings that were designed for more constant running.
The starter motors we used were very heavy. If I intended to build something to give a 'bit of a boost' to a bike, I'd try something smaller, such as a radiator fan motor (a 1" to 3/4" drum on it's axle would be the right size to give frition drive onto the tyre - we built an early 'Robot wars' robot with this type of drive to each wheel) or something from an electric scooter or golf bag caddy.
and work on the heart lungs. Motors pah 
