The original posting asked about using a resistor to reduce the voltage from 12 v to 6 v.
Using a resistor for the starter is a bad idea. The starter draws many amps when used and a resistor would "dump" many watts of power to change 12 volts to 6.
Example. Starter draws 100 amps. 100 amps needs a resistor of 0.006 ohms to drop 6 volts (ohms law). Power in resistor is 600 watts.
The best solution is to use a 12 volt field coil so that the full power from the battery flows to the starter without losimg power in a dropping resistor.
Or, just modify the bendix drive and use the original system.
Marc