Having lived in Duluth for five years, I know what sort of weather you are going to be dealing with. You will indeed want to get the window fixed, sooner rather than later.
Here's the thing. It's a fine line between draft-free and too-tightly insulated. You want there to be good ventilation in your coop, because ammonia fumes from feces can build up and cause respiratory problems. You will need to decide if you will be cleaning your coop out during the winter, or using a method where you add clean bedding all winter and only clean it out during the spring. Either will work, the key is to avoid the ammonia fumes.
What breed of birds do you have? If you're up in the northlands, you may well need to provide supplemental heat. Frostbite is a concern with Single Combed birds, and you'll want to ensure that the coop doesn't have those strong north winds blowing through it. Can you stack hay bales around the coop somehow? Can you post some pictures so we can make suggestions?
As Teach says, if/when you use supplimental heat, go with red heat lamps, white is not ideal.
Tell us more about your setup and we'll make what suggestions we can. And best of luck with them! I still miss the north, but not the extreme temps in winter, I will say.
