Could probably run the 6 volt motor for a while until you burn it up. The old 6 volt motors were built way better than anything new anyways.
Usually the case I'd the ground, and 2 other wires are low and high.
I'd probably lean towards running it on low with running 12 volts through it to aid in prolonging it's remaining life. 12 volts through a 6 volt motor will spin it up past it's normal low speed anyways.
Next spring, take it all apart, clean it up, spray bomb it, measure the dimensions of the 6 volt motor and look for a similar 12 volt replacmemt. Swap motors out (some fabrication might be required) and your all set.
With cold comes the need for defrost. Make sure you have some ducting to the windsheild to rid the fog of your hot stinky breath fogging up the glass when you first get in.