As Stambo said and Gerald linked, a brushed motor speed controller is the ticket. You may find you need to play games with your throttle curve. Also before ordering the controller take apart the blower and try to determine the motor specs. They should either be on the motor itself or try to grab some numbers off it and look it up on the world wide web (that's right, living in the early 90's here). Make sure you get current draw information so you get the properly sized speed controller.
Electric wheel chairs must use a speed controller of some sort. May be you can find an old wrecked one for parts.