Happened to notice this on ebay:
http://www.ebay.com/itm/41-5cc-Eton-2-cycle-scooter-engine-/270996543215
Looks nice, and the larger muffler ought to be quieter than the small canister type. It could work with a rear friction drive because it looks like it has a 78mm clutch bell, but possibly needs to attach a piece of pipe to the muffler to redirect the output 90 degrees so the heat goes downwards and not into your thighs.
http://www.ebay.com/itm/41-5cc-Eton-2-cycle-scooter-engine-/270996543215
Looks nice, and the larger muffler ought to be quieter than the small canister type. It could work with a rear friction drive because it looks like it has a 78mm clutch bell, but possibly needs to attach a piece of pipe to the muffler to redirect the output 90 degrees so the heat goes downwards and not into your thighs.