I was about to give you a bad time about your crude alignment job between the motor and transmission. But, if this is your idea of a low cost controller, alignment issues are the least of your problems!
However with this sense of adventure and only if it can be done safely, you might be game to be a tester. A lab rat so to speak. What about moving a heavy duty alligator clamp (like on the end of jumper cables) along a TBD diameter section of Nichrome or even steel or stainless steel wire/rod as a crude variable resistor. Maybe start with 2-3mm diameter X 30-40cm long. Be prepared for hot flying pieces of metal. Try it first outside the car with the wheels jack-up, like your first 12V test. VRs this size, rather than a straight rod, are usually a coil around a heat resistant core. With some trepidation, I say use your best judgement.
I've always wondered if this would be way to limp home with a dead controller, at reduced voltage, out in the wild with a brushed motor set-up.
