I have this really cheap block warmer from Target. I think women use it to dry their hair. Does wonders.
Maxxdout, there will be a threshold where you will no longer gain performance and quite possibly not see any kind of reasonable performance. That temp is somewhere below zero on the Farenheit scale. At that point, most RC modelers would rather be in doors by a fire or some other heat source. But as Pirata and others have mentioned, you should be able to find a nice performance mark if the engine gets tuned properly. I have run in all kinds of cold weather (not extremes, but below freezing) and have had great performance and great fun. It is do-able.