Most of the summer I've been riding a Trek 800 with a Thatsdax friction drive kit and Honda 35cc engine. It's been a reliable and easy setup, except when the roads are wet. The roller slips when wet. I recently bought a used Golden Eagle kit and I couldn't be more happy with it. It didn't have an engine but it did come with a heavy duty wheel with 105g spokes. You can buy these at Golden Eagle Bike Engines, Geared Belt Drive Bicycle Engines / Motors for Mountain Bikes & More
with either 2 stroke or 4 stroke engines. They also sell the heavy duty wheels. They are more expensive than the other kits but they are reliable and quiet. If you can afford it I think you would really like this kit. Personally, all the money and hassle I wasted on other kits I could have bought the GEBE kit from the start.