I would check with the manufacturer to see what they recommend. There are different kinds and models of heaters that may work great for one engine and not at all for another.
I live in the Northwest and most newer diesel tractors I have seen sold here have some sort of a plug in engine heater from the factory. Probably the most effective is the core hole, or freeze plug heater, which goes right into the water jacket, usually in the head. But unless you get the exact right heater, you may very well run into problems with the heating element touching metal inside the head, which will probably make the heater burn out very quickly.
On my old 641D Ford, I decided that replacing the lower radiator hose heater was the most practical way to repair the heating assembly on that model. I wish that they would last longer. The first one I had lasted 20+ years, but I seldom get more than 3 or 4 years out of the recent ones with the "safety" components.
If you have the right external coolant hoses (like heater hoses) it might be easy to install a percolator tank heater. They usually are higher wattage heaters and will heat up a small engine fairly fast.
A dipstick heater might help a little, but they are very low wattage, so the heating would be very slow. I also never liked the fact that they get the oil very hot right at their surface and always tended to get black deposits on the stick. Not too good for the oil, in my opinion.
When I was a kid, sometimes we helped start old flathead Chryslers by shining heat lamps on the carb and manifolds. Then I discovered that a headbolt heater was easy to install and cheap. My old Plymouth always started easily after that even at 20 below zero. I wish it was possible to get headbolt heaters for newer engines, but I don"t think they make them any more. They were great!
I have never tried a torpedo heater. As someone else wrote, I would worry about applying too much intense heat to the external surface of the tractor. If it was all cast iron, that might not be a problem, except to the paint, but on a modern compact tractor, there is a lot of plastic and other soft material. Heat can definitely damage lots of kinds of plastic. I would not do it, except as a last resort in an emergency. And then I think I would build a "tent" out of cardboard to direct the hot air over the engine from a distance.
I would sure try to figure out a way to be able to plug in your tractor for a while before you try to start it. If you heat up the coolant, my experience is that a diesel will start like Summer. It is also a lot easier on the rest of the electrical system than cranking over and over.
It is also important to use an antigell additive to the fuel if it is very cold. A gelled up fuel system will make a diesel hard or impossible to start, or will make it run real bad.
Again, I would check with the manufacturer or dealer to see what kind of heater works best with your tractor. Doing so might save you a bunch of time, money and effort experimenting with different heaters. Good luck!