Almost every aspect of fiberglass is repairable. You will need to make sure the mating parts line up perfectly, but after that it is simply resin, adding cloth for support, more resin, and getting everything to set up correctly.
There are a lot of 72-74 header panels out there for reasonable prices if you would rather not tackle a big repair like this. If you want to attempt it, you might be ahead of the game and save yourself some grief if you picked up a book outlining specifics of fiberglass repair.