Just a quick FYI on external heating and nose plugs.
I believe you will experience more efficient and quicker heat up if the external heat is placed towards the rear and low on one side of the engine (with the nose plug installed on that side) and the nose plug removed on the other side of the engine.
This causes a positive flow of hot air around the entire engine and allows it to exit on the unplugged side. This flow pretty much ensures even preheat.
Of course as mentioned in a previous post a insulated wrap is also used.
Cheers, very informative comments!