As said, the 6 volt starter is used on 12V here all the time. Keep it. Spend the money on a AC Delco 10si alternator and buy the alternator harness.
The time to switch to alternator is now. Do you know about the bypass/ballast resistor and coil and all that, for 12V? You need those parts also. The wiring sounds complicated, but it's not. We will help along the way. Lots of experience here on Stovebolt. Also, if you have a foot start, you may need to change the push button saddle switch, on top of the starter, to a side post style.
