I worked with a guy who's father was a logger in upper NY. In the dead of winter he said dad would just start a small fire under the skidder in the morning. He said those Detroit Diesels were bullet proof but when the temps drop far enough it was either set it on fire or not work that day. I have heard of people using sheets of plywood and a space heater to blow hot air under a tractor to get it going. For me, if it's that cold, then I try to stay inside.
70's winters were typically colder than today's. I remember a news item about 2 guys stuck at a cottage in Northern Quebec during a holiday cold snap. Couldn't get their Civic to turn over, oil was too thick. Rather than wait for Spring, they loaded up a Hibachi with charcoal, and bbq'd the oil pan for an hour - worked great.
Reading down this page (that's some cold MHarry), I was thinking tarps + salamanders, but Egon beat me to it.
I've been liking the propane catalytic heater I picked up last Winter, but so far haven't needed for equipment starts..... fingers crossed.
Rgds, D.