My apologies if I seem unnecessarily critical, and of course your property is yours to do with as you wish. Obviously I hate to see an original Elec-trak deck motor get used on a 120 volt application if there's any alternative; I'm envious of anyone with spare 36vdc stuff since it's so much harder to find laying around than 12, 24 or 48 volt DC equipment (and 120vac motors are common as dirt). Call it jealousy!
In re: variacs, I've read thousands of (often conflicting) words on this subject since I last posted. What I learned is what I *always* discover whenever I start looking at electric motor characteristics - that it'd be a lifetime's work to understand everything. Variacs are fine for speed control of induction motors specifically designed to work with high slip under consistent loads (such as certain fan motors, and presumably my forge blower) and are more efficient overall than rheostats and some types of sold-state controls, but the AC motors they work best with (PSC and Universal) don't seem appropriate for the torque and load requirements of a metal-cutting lathe.
Surprisingly, variacs apparently make great DC motor speed controls, as long as you rectify the output, since most kinds of DC motors don't really care much about ripply supply voltage. But I think the great thing about variable autotransformers is their extreme durability for the price - you can get a 10 amp variac cheap on eBay and they'll last much longer than an equivalent solid-state device. People claim to have used ohmite variacs around the clock continuously for decades without failure.