if you want to do a pwm type of deal and alter the duty cycle555lm cmos timer from radio shack. You can reduce the voltage going to the chip to logic level (+5v) with the good ol r-shack voltage regulator which would eliminate the need for a ginourmous resistor. You set the freq by placing resistors and a capacitor across a couple of pins. I used one recently in a voltage multiplier but the concept works for reduction as well. You will need some sort of transistor drive to handle the current going to the motor via a mosfet , you can setup a totem pole arrangement (non-inverting) with an npn/pnp combo of transistors which can be used to drive a mosfet(s). If you want to google non-inverting bi-polar totem pole, you can get a schematic for the driver. The mosfets you just wire in parallel until they handle enough amperage, 3 irf510s (availible at radio shack) should be enough.