For my C55 conversion, I worked with Eldon (the Motorman) to come up with a good solution to the motor problem. As-is, the motor runs very fast. I would estimate the top speed to be about 200 mph with the stock motor.
I purchased a Faulhaber 1016 coreless with an 8mm gearhead, then mounted the worm to the motor shaft. The motor does not stick out of the back of the Minitrix shell, and with a good decoder, the loco will creep along and have a top speed of about 80-100 mph.
I highly recommend this upgrade. There is some frame milling that is required around the area where the rubber motor linkage is located stock.