It's not something you really need to get fixed immediately (unless the fumes are coming into the vehicle). The biggest problem you'll have is a lack of power. The previous owner of my mustang drove with the exhaust manifold cracked for close to a year before I got it. If you can smell the exhaust inside the vehicle then you either need to drive with your windows down or round up some money asap. It's terrible on your health to be inhaling the exhaust fumes and it can lead to death. If you can afford to replace the cracked header, take it to a muffler shop and see what it would cost to have it welded up. Not a permanent fix but with will suffice until you can afford the new headers. That's what I did with mine because I plan on getting long tube headers, possibly in the spring.