Regardless of the source of engine "pre-heat", it all comes down to one thing, TIME. That heat needs to propagate all the way to the crankshaft to achieve the desired objective. My analogy is thawing a frozen turkey.
As Denny stated, the simplest (and safest) solution for engines with electrical heating systems is the compact portable generator. Get one with the longest run time under rated load because to be effective, it needs to run all night. My 1800-watt generator runs near 7 hours with the draw of a full Tanis system, less if I also run a small ceramic heater for the cabin interior. I run a 50' arctic rated cord out to its full length which is far enough away for me to be comfortable leaving it while running. If you need more assurance, use a 100' cord, just ensure wire size is adequate. Another advantage is no need to carry fuel, its already in the aircraft. I have been running my generators on 100LL for coming up on 3 decades (chainsaws as well).
As for combustion heaters, they are my least desired heat source for pre-heating an engine. You cannot turn your back on them, which leads to impatience and terminating the heat short of its goal with the net result of the engine not being properly heated. Save the flame throwers for making coffee, the AC source is not only safer, but more effective as well.
TR