There are two cases here, one with the old six volt motor and one with a twelve volt replacement.
If you are using the old six volt motor than an external dropping resistor of about 1 ohm and a rating of at least 40 watts will work with the old motor and old switch.
If you are using a replacement twelve volt motor than a new twelve volt switch would probably work best. The variable resistor in the switch is sized with the charactersitics of the motor in mind and would be different from the six volt version.
