If you purchase a three speed motor and a DC motor speed control unit from eBay, you can have fully variable fan speed through the use of a rotating potentiometer. wire the speed control to the high speed motor winding. I did this on 2511 years ago and it works just fine.
if you gut an original rocker switch you can mount the potentiometer in a manner that makes it almost invisible
replacing the heater indicator light with an LED bulb will allow that bulb to illuminate even at the lower speed voltage
Larry