Only one suggestion, GEBE.
Golden Eagle Bike Engines, Geared Belt Drive Bicycle Engines / Motors for Mountain Bikes & More
Of course it is a heavily opinionated suggestion.
I am really impressed with the dependability and durability of the kit. I have read many owners evaluations on them at this forum and a few others.
They are nearly as easy to set up as a friction drive system only they have an all weather advantage.
I plan on getting one as soon as I can afford one.
Do your research carefully and enjoy this place!