I looked at one of those (Honda 2-cycle) at the beginning of my quest for some motor power on a bike.

My problems with that situation included the folowing:
1) Too noisy. Can't conceal the motor noise while using it on a trail.
2) Too stinky. The motor releases gas fumes like a lawnmower. Somewhere along the way, I was told that I would arrive to work stinking like gas fumes.
3) Still have to pay for gas, which is more expensive than electricity.
4) Not environmentally friendly. The emissions from one of those in one hour surpass the emissions from my car in a week. These are actually lawnmower motors.
5) Just as heavy, if not more, than a decent electrical motor.

I would put up the investment for a decent electric bike, either a simple e-bike or getting an electric motor to fit the one you have. (I considered putting Bionx on my Beach Crusier, but it made more sense to put it on a more efficient bike. Still using the Beach Cruiser for short commutes, though.)