The motorized kit that some have referred to is not ideal.
It is 2 stroke. A 4 stroke would be much better.
It is very noisy. Anyone putting one on a bike should look for a better exhaust, probably from a motorbike.
It has no gears, and is like riding a motorbike in 2nd gear all the time. To go at a decent speed, the motor needs to rev hard, which no doubt shortens the engine life. It would be ideal to have a smaller rear sprocket, so it can go faster without revving too hard. Then the rider should start with pedal power only, to avoid killing the clutch. The motor is then started by dropping the clutch.