The easiest way to convert from 6 volt to 12 volt is with an alternator kit. This includes an alternator with built - in voltage regulator, alternator stand, alternator clamp and 12 volt fan plates. Unlike other kits on the market, there is no core charge on this alternator, which means less hassle and cost. You will also need to change your coil to 12 volt.
You can handle the starter situation in two ways: You can continue to use your 6 volt starter and flywheel or you can install a 12 volt flywheel that will require a 12 volt starter and 6 volt to 12 volt adaptor bushing. To do this, you will also have to grind out the bellhousing for the larger 12 volt flywheel. This will mean you will also need a 200 mm clutch assembly.
A rare exception would be if you should happen to have a 6 volt 200mm flywheel and clutch. If you do, we recommend using your 6 volt starter and 200 mm 6 volt flywheel. If you have a 6 volt wiper motor, it can be used with a resistor to lower the voltage and slow it down.
You will, of course, need to change your battery and light bulbs."