You can use a heat gun, preferably a hair dryer to heat the engine head on really cold days. Battery-powered RC Tire warmers wrapped around the head work nice for keeping your engine head warm during down-time.

On very cold days you can block off a portion of the venting on cowled engines to help regulate engine temps or you can wrap about half (speaking vertically) the cooling head in heat resistant foam. Just keep an eye on the engine temp after a short first flight to be safe.

Here is an old, but solid article for RC car engines but most of the techniques translate. It reiterates what I've said to a point but explains more.