AC Motor Speed Controller for Brushed Motors - 120V 15A Variable Speed Control with LED Voltage Display & Overload Protection, Fan Speed Controller for Duct Fan, Exhaust Fan, Router, Drill

Looking at some forum posts, this is a brushed motor, so any basic Triac based electronic speed control will work. Try some cheap options to start, if you want a nicer one they're available for more $$$.You'll have to hack it together a bit so you have access to the knob, but you could probably use the existing knob (it's just a potentiometer) with a little soldering. Then just wire it up in line with the existing start/stop switch.