This is more, and thus heavier, than needed but again is about the smallest available. For my assists I decided to use a Honda 22cc four stroke string trimmer motor. It is a little heavier, and more expensive, than similar two stroke engines. So far I've been very happy with the choice. They both use a timing belt speed reduction and a friction wheel to apply power to the rear wheel.
Most commercial units run a friction wheel from the motor crank shaft directly to the rear wheel.
This gives good top speed on the flat but lugs the motor at hill climbing speeds. Since the purpose of this assist is hill climbing a reduction is needed. This also allows a larger friction wheel to be used which should lessen power losses. The picture in Figure 1 shows the motor and reduction drive removed from the bike. The unit mounts to the bike by the two rods end links and the shaft seen to the right side of the picture.