Extension cord and hair dryers. Use 3-hair dryers for preheat at my tie-down. Total cost was $33 at Walmart. Two in the inlet over the cylinders and one down below aimed at the oil pan.
If not possible with electric, I use a small dual fuel, singe burner camp stove.
To direct heat into cowling at the exhaust opening, use a 6" x4" HVAC duct reducer into a 4" dryer hose. A short piece of 6" duct around the stove clears the height of the stove and then the reducer necks down to the 4" hose. The hose is aluminum and expandable. Small light weight and portable. Holes drilled in bottom section for air flow.
A small primus fuel bottle and I have enough fuel for 3-hour warm up.
I would use caution if using propane, have had the moisture, from the burned propane, freeze the throttle cable in the past. (0 deg F and below).
Best regards,
Mike Bauer