That's a fantastic model! There's a hell of a lot of crap advice in this thread. You have two main options, both of which have been mentioned already though: 1. Steal the rheostat (high-current potentiometer) from a fan controller. You'll need one with a high power rating and reasonably low (<1k) resistance to drive your motor, as it sounds like it's a much higher power motor than the ones used in computer fans. 2. Get a PWM based fan controller. This is probably your best bet, as it's less likely to burn out and is much more energy efficient. Again though, make sure it's rated at a high enough current/wattage for the motor you're using.
You can make a simple PWM controller with the 555 timer IC and a few other passive components. You could also use a potentiometer and a suitable transistor to create a very basic controller. Potentiometers only deal with very low current and burn out so easily with any sort of motor. A transistor can sink larger current but will work off a very small current. I've had such a circuit working on a PC fan with a 200mA transistor. I think it was a 2N3904.
