One option is to call Casita
. They still make that model, and as far as I know, the molds haven’t changed. They sell fiberglass parts, but they aren’t cheap, and shipping will be expensive due to size.
The other is to fix it. There are a number of old threads on door repair to guide you. It typically involves rebuilding the core to restore proper curvature. The “site search/google” option at the bottom of the search menu is the best way to find them. There’s also a long thread on how to do fiberglass repair work, which will be required to fix the door.