6v to 12v step up converter

I'm sure, SOMEWHERE there is or was 6V inverters to 12V. There used to be such accessories available for 2 way radio installations, esp in such as snow plows and fire trucks, etc.

OLDER small motors are not permanent magnet fields. Older small motors, even such as used in our A bodies, are all electro magnetic, or wire would fields. These motors do not care about polarity. Such is true of the older cars on starters, or any small motor in the car

You might try to find an older 12V motor to swap in. An inverter is likely to be expensive.

Yes!!! Newer PM (permanent magnet) field motors WILL revers if you reverse the lead wires.

Also keep in mind that your 6V generator is likely VERY much limited