So while I had the garage open today to get some toys out for the girls to play with while we enjoyed the sun (and shade) in the back yard, I took a closer look at the treadmill. Found the motor controller! It’s mounted to an aluminium plate that is attached to the steel frame with two Phillips screws. I’ll unplug it once I have time to trace back all the wires that plug into it so that I can label everything. That’ll make it a LOT easier to get the right parts when it’s pulled out and put into/on the go-kart.