YOu can break it in now, but it will not be a pleasant experience at all. I usually don't do break-in's if it's under 50F outside. If it's colder than 50F, it's tough to keep them warm, to not cool too quickly and to get them started. The majority of the reason for break-in is to heat cycle the engine up to 200F-240F and let it cool back down to around 70F. This allows the sleeve to expand to the proper size and allow the piston to wear into it properly.
If you have a heated garage, then I'd suggest starting it in there after heating the engine up with a heat gun or typical hair dryer. A hair dryer will usually get the head temps up to 150F or so.
You should at the very least wrap the head with a paper towel and ziptie it on. This will help keep heat in. If your garage isn't heated, or you don't have one, you really need to get the engine up around or above 90F before starting it or it will be very difficult to get it going. Also, if your garage isn't heated or you don't have one, you can wrap chemical hand warmers around the head and ziptie them on. Make sure to shake them first to get air into them to heat them up. They should keep the engine from dropping below 70F or so, depending on your ambient temps.
Even with all that, you more than likely will cut the life of the engine quite a bit overall.
