Re: restoring an old boat
That hull wouldn't happen to be avocado green, would it? I dumped a mid-70's Chapparal 15' tri-hull a couple of years ago. deck and transom was completely rotted out, and not worth the restore.
What kind of motor does it have on it? If the mechanical aspects of the boat are good (steering, running motor, control, etc) my suggestion to you would be to strip off all the usable bits and trash the hull. Go find yourself something in servicable condition that will match well to the motor you have and transfer all the mechanicals to the new hull. It will get you on the water this season as opposed to some time next year.
Barring that, if the deck and transom are in good shape, I would suggest that you DON'T put the astro-turf carpeting back in there, as that will trap moisture. My solution was to get high-quality porch paint with a traction additive from the local big-box store, and do the floor with that. I was two seasons of fishing and summer tubing with that floor, and it still looked new. Infinitely easier to clean than carpeting. Since the boat was used for fishing, I really wanted to put something in that could be hosed down and wouldn't trap pieces of bait, guts, blood, etc.
As to the seats, best solution is to build your own. Fairly easy to cut and upholster. If you do so, use properly sealed marine plywood and UV-resistant vinyl. If you need the hinges, you can probably find them online, or you may be close enough for a day trip to Lanier Marine Liquidators. They have a boat graveyard that they will allow you to come and pick parts. You will have tons of old tri-hulls to choose from to get those bow seat hinges.
Good luck.