In our 1940 Chrysler, for things like wiper motor, heater motor we used a 12 volt to 6 volt transformer to pad down the current. Found on ebay, can get a pic for tomorrow if needed. The starter draws a lot of current, and I wouldn't use a transformer, but should be able to handle 12v.

In our case we installed a 12v wiring and changed all the lights etc, but couldn't find replacement electric motors in 12v.

Mike