Hi Les,
You can convert to 12 volt using the original generator. I have a Special that uses the standard Miller generator wired to give 12 volt and incorporating a Zener diode to dump any excess voltage to earth. Although only rated at 60 watt, when set up for 12 volt, it gives in excess of 8 amps at 12 volt (96 watts). Of course, when operating like this, the generator is giving full output, so needs more power to drive it - not good when you have a Dragonfly with just 18 BHP!
To get 12 volts, firstly replace the original rectifier with a modern solid state unit. The generator lead that went direct to the old rectifier should be connected to one side of the new unit. The other 2 generator leads that went to the lighting/ignition switch should be joined together and connected to the other side of the rectifier. The Zener diode should then be wired into the output side of the rectifier (to control the maximum voltage).
Regards,
Eddie.