Staton has a well built friction kit; although the kill switch and thumb throttle remind me of the quality you'd expect to find in a cheaper chineeze kit.
Knowing beforehand that my 25cc/.93 drive roller choice wouldn't break any speed or power records, I went with an engine onhand just because I wanted to try propane. Guess maybe King Of The Hill got to me.....
While this is shown on a Schwinn, it is now installed on a Trek Cruiser and that's a great feature of this kit as transfer from one bike to another is minimal effort.
Update: Just remembered this pic of the TREK.
Intergrated a concealed kill switch into the faux tank panel and used a GoPed thumb throttle to clean the kit up a bit. Have a center stand on order.
Have another 2 stroke weed whacker engine setting out in the garage that I hope to build similar to a BumbleBee just to do it. Unlike the Staton, don't think I'll care that it'll need manual engagement/disengagement anytime I stop.