About forty years ago there was a thing called 'the Bell Conversion' which did exactly that. I fitted one and used it for several years. It had two idiosyncrasies. One, you has to rev it hard to get the cut out to cut in, after which it was fine Two, at low revs round some of the country road corners where I live it would not keep up to the demands of the headlight and coil ignition. One evening I was returning from a Club Night with Bill Turner on his open 'D' behind me. Bill had fitted a car alternator to his bike. On the low speed parts I could not even see the light on the road from my bike as it was totally overwhelmed by Bill's headlight. Later, Dick Sherwin and I developed what has become known as the 'Walkernator' and despite the non standard appearance many people are happy with it.