If it's the fabric itself that's falling apart, could you get a bolt of black/grey/beige/whatever color from a fabric store, some double sided tape (or glue), and maybe an industrial stapler? I'm thinking:
1. Take out the headliner that's falling apart, including whatever foam backing that's there
2, Get a piece of cardboard from whatever your last large purchase/delivery was, and cut that cardboard down to the same size as the fiberglass roof panel
3. wrap the cardboard in the fabric that you purchased, using double sided tape/glue on the passenger compartment facing side, and stapling around the edges where you've wrapped the fabric to the other side
4. Install the fabric-wrapped cardboard in the ceiling - I would imagine you'd want to use something like epoxy to make it more secure.
I think this would work reasonably if the headliner area is reasonably flat, or at least doesn't have much in the way of compound curves.