I use a 200K BTU torpedoe heater with diesel in my barn or in my shop as necessary. I have found a couple of useful things:
The heater has to be big enough to heat the area with doors open. I want to stay alive.
You don't have to heat the whole building, just an area to work in. I hang plastic tarps to 'wall-off' my work zone. Pay attention to the ceiling sealing. That's where the heat leaks out.
Its not the air temperature that's important, but the temp of the machine I'm working on. That means warming the tractor mass up if that's the job being done. Just point the heater at it and wait a few minutes. nice warm fenders or tires to lean on is much more comfy than stone cold ones.
Warming all the tools you will need is the last necessity. That means you don't need ski gloves to change out a starter or a battery or a belt or whatever. A freezing cold box wrench in my bare hands is a ticket to quit.
If you don't have a big fuel heater, a very high wattage light can help you almost the same, plus it helps you see better in a dark building. Just don't look at it.
Don't overdress. That insulated suit will keep the warm air out of you just as well as preventing cold from getting in. And by all means wear a warm hat, one without a visor. The visor is the first thing to bump into your work that results in your head getting cold when it gets knocked off.
Wear safety glasses, but wipe them with dish soap so they don't fog up when its time to breathe with your face right up against that thing you are working on.
