Before I got the Felt, which is a very expensive cruiser, I used two friction drives on Trek 800 steel framed mountain bikes bought off Craigslist. The 800 is a super strong bike, with strong Araya rims and wide flange hubs. It did require a few modifications to suit my needs. First was a much larger cushier cruiser seat and a suspension seatpost. Then much higher bars that I could reach straight out to instead having to bend over. That required longer brake cables. I then removed both shifter cables (no reason to replace them as I didn't need them) all but the smallest front chainring, put the chain on the largest rear sprocket, and adjusted the derailleur to stay put. It became nothing but a chain tensioner. Many people say not to remove the gears, but in my case it made no difference, I can't pedal anyway, other than to help get started, and I need the lowest possible gear for that to keep stress off my knees. My new Felt has no front gears, and because of it's price, I chose to leave the rear gears alone. Because I might want to sell this bike at some point, I cut and glued heavy pieces of rubber to the front engine mount brackets to prevent damage to the frame. Not necessary with an older bike. The main thing is the rear tire. It needs to be a 2.125, and fairly smooth. Not an off road tire. I've never had problems with a rear tire. I am also using a GX35 engine. It should last a long time, and is quiet. I don't plan on going over 20 mph either, really 10-12 mph top speed would be fine. I don't plan on racing cars. I ride it the same way I used to ride a pedal bike.
As nice as the friction drive kits work (also rear chain and belt drive kits) I don't even know why anybody uses those crappy 2 stroke Chinese engines and deals with rag joint sprockets and killer chain tensioners. Price and speed seem to be the main reason, plus they want their bike to look like a motorcycle. All that does is attract cops. All I wanted was a reliable bike I could ride at bicycle speed without pedaling.