Diggin It, you could always "throw good money after bad", and add one of these
Router Speed Control
Altho I wouldn't recommend it - actual vari-speed drills tend to have more elaborate circuits to keep more torque at lower speeds than those old-fashioned SCR controls.
I WOULD, however, recommend that speed control for a simple incandescent dimmer - I've been using a dead upright freezer in the shop for several years as a dust-free tool box for some of my nicer woodworking tools, etc - I put an "outhouse" style ceramic bulb holder in the bottom of the freezer, ran a cord thru the drain hole, took the "mushroom" drain cover out and drilled a hole in the top of the freezer, installed the "mushroom" there (small vent, but covered) - I started off using a 100 watt rough service bulb, but it got too warm and burned out in a month or two. Had to check it every time I went in the shop, PITA...
I had one of the controls in the link, put that on the cord to the lamp (outside of the freezer), turned it down to about HALF brightness - same bulb for the last 4-5 YEARS. Temp inside the (dust-free) box stays around 80 degrees even when my (mostly) un-heated shop is in mid 30's. Absolutely no rust on ANYTHING; a combination of near hermetic seals AND never below dewpoint temperature.
And no, the dead freezer is NOT a HF tool; but I'm a sucker for a USEFUL re-purpose, and this one IS.
I now have 4 OTHER freezers (free, on CL) in a couple of my shipping containers - no power to any of the containers yet, but they keep any "heat sensitive" stuff from hitting 150 degrees in summer sun... Steve