This is an old thread, but some of the info needs updating.
The best controller for a treadmill motor (IMHO) is the MC2100. You can get a signal generator for about $10. Set it to 20Hz, and the duty cycle varies between 20% and 80%. The duty cycle controls the speed. Using a handheld tachometer, I made a chart of duty cycle vs spindle speed. Keep the pulleys, I have both a low and high spindle speed. Stays on low most of the time, with a max spindle speed of about 1,000rpm.
The connector that the signal generator feeds into will also provide power for 12V LED lighting, and I'm still investigating using the incline control for a powered head lift.