I'm pretty sure they're solid fiberglass construction (without a foam core) - just like the Nacra 5.2's of that era.

Repair is not going to be terribly easy but it can be done (anything can be done with fiberglass). I'm pretty sure the deck is similar to the hobie 16 style - that it is a solid piece that has a lip that slides down over the hull. If the soft spots are large and wide, you are going to have to get large scale access to the inside of the boat...that means either cutting large access holes in the decks to be repaired later or removing the deck entirely.

You would then glass in reinforcements and/or horizontal stringers to beef up the aging fiberglass from the inside. This is not an easy (or terribly inexpensive) task. I'm sorry to say that you would probably be better of searching for a newer boat that you could reuse most of your gear on.