Replacement part for coal stove blower rheostat
That looks like a standard A/C dimmer switch circuit:

http://forum.allaboutcircuits.com/threads/potentiometer-w-switch-wiring-for-lamp.79664/

I'd imagine you could replace it with a standard A/C variable motor control like this one:

http://www.globalindustrial.com/p/hvac/exhaust-fans/in-line-duct-fans/kb-electronics-vari-speed-ac-motor-speed-control-kbwc-15-solid-state?infoParam.campaignId=T9F&amp;gclid=CN-z8pGMtsoCFYM8aQodWAoOaQ

The 4-wire versions require you to connect the white wires in your box as well.

Alternatively, Harbor Freight and other sellers sell a replacement for this fan controller here:
http://www.harborfreight.com/router-speed-control-43060.html

All it is is a dimmer switch in a box with an extension cord.