You could just replace all the speed control circuitry with the ebay device, and use a stroboscope disc and just draw lines on the pot scale where 33/45 revs are, if absolute accuracy isnt required this oughta work.
I'd be looking at the pwm controller and finding out what freq it runs at, you'd want it to be well above audio or else you'll hear a whine from the speakers, and even this might not be great if your feeding very high power amps with this thing.