The Solex propulsion system used a centrifugal clutch, and a roller on the front tyre. The operator pumped the pedals a couple of times to get moving, then opened the throttle. I can recall them being modestly popular in Amsterdam in 1971, but only with middle-aged women. The men, and especially the boys, were into mopeds that were really disguised 4 speed motorcycles. (The rules required less than 50 cc, and having pedals that could drive the bike along. Mopeds did not require registration or a driver's licence in Europe.) The Solex was not sold as a power unit, only as a complete moped as far as I know. It was a very heavy machine compared with a push-bike. It also had a wide, flat-tread front tyre. I don't think it will be very successful on a push-bike - I put a 75 cc Tor with a roller drive on an ordinary bike's front tyre when I was at school, and it ate both the roller and the front tyre in a few miles. Also, you could see the front forks deflect when you opened the throttle.
