Back in the days before global warming, when winters meant periods of
-30 to -40 weather, and block heaters failed at the most inopportune
times, I occasionally used a coleman stove under the sump of my
Vauxhall.  ( Trying to run an English car in the depths of a Canadian
prairie winter is a whole story in it self).  Worked fine. I also
used to place a trouble light with a 100 watt bulb next to the block
and throw an old feather comforter over the engine. Started easily in
the morning.

Grandpa had the best solution back on the farm. He'd park on top of
the manure pile. The heat from "composting" was enough to keep the
oil fluid etc.