I have one of those magnetic heaters, I used it for a old backhoe that was on my property. Also, I have a kerosene heater (for display only) that is intended for heating the oil pan.It has a mesh cylinder that I am guessing is sort of like they old Coleman lanterns. You were supposed to light it and put it under the oil pan. Never fired it up.
A coleman lantern (Both liquid fuel and propane) would work as a indirect heater. Collectors often use them to both lightup and heat backyard sheds. The vent top would be a shield for the flame and the mantle.
You could also throw a tarp over the front of the truck and use one of those Vevor diesel heaters to blow hot air at the engine. It is similar to how people down in Antarctica warm up the dozer engines so they will start.
