Looks like it was coming apart before the crack by the buckle in the hull. I’d take a jig saw and cut away the bad glass plus about 5” on each side in a rectangle shape. On the other hull, the good one in the same location prepare the hull with duct tape then wax to use as a mold. The hulls are pretty much symmetrical so the patch from the outside of the good hull will be for the inside of the bad hull……….so forth. I would also use Epoxy and not poly resin. I don’t recommend mat - no strength, use a biax and make one or two lay-up patch with about 4” larger than the damaged area. This will be your patch panel. I’d make 2 or 3 pieces that is a port , starboard, and deck patch.. Make one patch panel for each side and one for the deck. They will bend some so you can fit them inside the hull. Scuff the inside of the hull to get a good clean bite. Make sure all the wax is removed from the molding process. Clamp the panels in place and let dry. Also inspect the inside of the hull for any other damage before you seal it up and make sure bulkheads are attached to the hull. The seam is splitting to it needs to be ground smooth, filled and glassed over from the inside, and outside Taper the outside of the repair area then you can glue in foam or make a solid patch, and put your outside skin and finish. . West system has a short video on the repair of foam/fiberglass that covers the basics http://www.westsystem.info/wesyephodvd.html . I have some pictures of similar repair not as big and I used the cut out piece but the same idea http://themanshed.net/archived-projects/g-cat-catamaran/?g2_page=2 and http://themanshed.net/archived-projects/g-cat-catamaran/?g2_page=3
On the Supercat 20 I had to make a piece for the deck where a funky wood top patch was made. I made a temp mold of the other port hole and deck area, had to spin it 180 and correct the angle. http://themanshed.net/archived-projects/supercat-20.html Since you are close to the bulkhead make sure you do not cut into it and you can tie your patch into the bulkhead to make it stronger. Back then I used bondo type products for faring and have since removed all of it. Do not use plastic fillers they asorb water, it is best to make an epoxy paste with a filler it is a little harder to sand but much better.

I would not use the old piece of the SuperCat15 since it was failing and check out the skins around the cut-out to make sure it is sound.

It is getting late I worked too late in my shop so I need to turn in if you would like more detail you can contact me directly.