Finding information easy to understand can be a challenge because often websites assume a level of understanding that someone new to the game just does not possess. Others with more knowledge might want to chime in because I have no personal experience with freewheeling hubs. I believe Staton Staton-Inc motorized bicycles bike kits and gear drive kits.
offers a freewheeling Hub. The GEBE setup offers a belt system that they claim is very easy to remove and replace the belt from the wheel Golden Eagle Bike Engines, Geared Belt Drive Bicycle Engines / Motors for Mountain Bikes & More
. Also, I believe the crank that comes with the transmission kit from Welcome to Sick Bike Parts
gives freewheeling capability to a china fire kit, as well as the ability to route the engine through your bicycle gears. There may be others, also.
(edit: I could be mistaken on the kits mentioned, someone feel free to correct me if so)