When I was in the aviation business, we installed Tanis brand heaters on just about everything. Of course most all our engines were air cooled, so no water radiator. But still, there were plenty of 0-F or below zero (F) days/nights when starting without a Tanis system proved impossible. 

On most of the engines, the Tanis kit had glue-on (silicone) pads on the oil pan and on the topside of the crankcase. Some kits also had heaters that would screw into the cylinder fuel injector ports, assuming the engine was not injected, but had cylinder heads drilled for it.

The Tanis kits usually didn't draw a lot of wattage. If you had a twin engine airplane, you could use an extension cord to hook them together. Always carried cords (cold wx flexible) with you so you could plug in at airports. 

Without Tanis heaters you were just lucky if you could get some engines started on a bitter cold day. Plug a Tanis in and you were guaranteed of starting. We used them on engines from 200 to 550 c.i.d. But, it was important to try & limit the heat loss while plugged in. Sometimes used cowl inserts, sometimes blankets, or just get out of the wind somehow. Again, these were air-cooled engines.

A sometimes cheaper alternative to the Tanis was just a magnetic heater for the oil pan, assuming it was steel. I would use a magnetic heater or stick on pads on a truck if constant cold weather starting was necessary. Since the heater pads were relatively low wattage, there was almost 0 chance of fire, unlike concerns of a water block heater. 

Link to Tanis aircraft heaters#mce_temp_url#