It is quite common that the cardboard shell is shot. Mine was heavily cracked after I wrestled it out of the cabin, and what I did is gluing and stapling reinforcement parts from cardboard shoeboxes all around the edges, on the outside of the shell (the part that touches the roof when mounted). I did one patch a day, so it took me about a couple of weeks, but in light of the fact that there are virtually no shell replacements available, it was worth the effort. For a nicer appearance, I bought nice, 1960s' headliner material with correct foam backing from SMS in Oregon, had my interior guy glue it on to the shell (which I sanded and vacuumed thoroughly).
There's one alternative though: some company offers a kit with header bows that mount underneath the headliner moldings, meaning you don't need the shell. Don't have first hand experience with those but looks like a wonderful solution.
Search the Internet.