The magnetic heater will be the quickest and easiest to install, but as others have said, some sort of coolant heater is best.
Back when I was growing up, there was a long cold spell that lasted a while being below 0º, nothing on the place would start, my dad had used the white gas camping stove under one of the vehicles until it was warm enough to start. Amazing what multi weight oil, fuel injection and better battery technology does in the cold.
I know that open flames, oil, gasoline/diesel fuel and other flammable materials are very dangerous together, and I am not recommending doing this, but it did work.