Is that motor geared? That puts so much resistance into the potential windmill that it can be frustrating.
You might want to poke around e-bay for "stepper motors" and "tape drive" motors.
You should do a bit of reading, on this site, and from sources like Hugh Piggott, about the way windmills work. My gut feeling is that a windmill with a 3-foot diameter prop will want to turn very quickly. This is fine, but if the motor you put it on is supposed to run at 200 RPM, the two parts are badly matched.
At a guess, a prop that small should run about 700 RPM when at its rated power, and given the size, the maximum power would be about 50 watts or so in a 20 mph wind.
If you can find an appropriate motor to convert, it is a great starting point. There may be threads on this forum that can give you hints.
