Draining oil is a start and addresses the oil, but doesn't address the need for preheat, with or without having prepared the oil.
Well, it will obviosly vary from engine to engine, but a bucketful of
hot oil + some careful cranking may be enough to bring the engine to its starting temperature if it isn't very cold outside.
BTW, speaking of external combustion heaters... Just a couple of weeks ago, a Mi-8 helicopter in Russia was destroyed by fire after an explosion of the ground-based heater. If you use one, always have a fire extinguisher within easy reach.