I found an old thread in RAH about this and one poster said he had had
moderate success with aeromodeler's light fiberglas and Jet super glue.

He would glue the glass to the back of the broken or cracked plastic,
reinforcing it. Anybody else had any success at this?

And then I found where a member of EAA Chapter 9 in Columbus, OH had used
fiberglas to repair the plastic interior parts on his Cessna 172, but not
enough detail at the website, except for the neat ideas this guy, Wetherald,
had for dyeing and repainting the plastic.
http://www.eaa9.org/special/wetherald_interior.htm

BTW, you can get $3 thousand tied up in interior plastic panels and trim for
an airplane in two shakes of a cat's tail.

As for the carpet and upholstery I took out Airtex, which was excellent
quality but worn and replaced it with Hangar Bay, which though new is poor
quality. If you go to Hangar Bay's website, don't be fooled. The interior
shown there looks much better than it really is. The owner, Jerry
Robinson, shows the fronts of the front seats, so that you are unable to see
his cheap treatment of the rear of the front seats, which have no map
pockets. The carpet, unlike an Airtex carpet, is not bordered or lined,
will unravel, and is generally poor quality. On top of that is is cut
poorly and is a poor fit. Good luck to you if you deal with this guy; you
have my sympathy.