Yes I did do a FD with a four stroke. It was a R/S 35cc on a Staton Kit. I sold it for project funds. It worked really well slipped a little when real wet. I
could baby it along in bad weather. 22- 25 mph with a lightweight bike and myself at 155lbs.
The kit I have know I call my fair weather bike. It is difficult in the rain, I think it is do to the pivot point on the mounting. I drilled new holes toward the center of the back of the mounting plate. The original one made the motor sits way up and I'm to lazy to make different length bracket. The 2 stroke motor is strong and pulls me along at 32mph, fast enough for me.
The Atlas frame mount HS 4 stroke is the one I ride. My favorite right now. If your looking for simplicity the FD kit can be put together in an afternoon or much less. If you do go with it a excellant add on is the 96ounce tank from Staton well worth the price for more riding instead of refueling.