Think about this, you have old wires, an old switch and an old motor. Without doing it 1st class best option is place the resistor before the OLD 6V switch and give it a break too. Mount the resistor away from everything else due to heat.
Regulators are ok, just another piece you have to scab on to make things almost right.
Best way was suggested above, buy a 12v motor, also get a 12volt switch, put in new wires and never worry about where to mount a resistor, regulator, fight an old switch and an intermittent motor.