ouch!

problem is that if the port hull is failing internally the starboard hull could be too....

so assuming you just want to get this sorted quick so you can keep sailing this season

how about
-first check all internal bulkheads and stringers as much as possible by looking in the ports on both hulls
-add extra glass cloth and epoxy resin if needed to any delaminated areas
-press the damaged bits of glass back into their holes and put large overlapping glass cloth bits over them, outside first then inside. keep the outside as smooth as possible by turning the hull on it's side, covering the fresh repair with plastic kitchen film, then newspapers, cardboard and old magazines so it all sets as smooth as possible
-when holes repaired, make some cardboard templates for bulkheads, 1 for just ahead of the failed cross member and 1 for just behind
-once shape sorted, transfer to 4-5mm ply and use epoxy resin to glue them in place
-after epoxy set, add 1/2inch fillets around bulkhead circumference, use a filler like carbosil? to thicken the epoxy or it will all run off
-when fillets half set, add 1-2inch glass strips on top so that an equal amount of glass is on hull and bulkhead
-when everything set, add a final coat of resin on top of everything you've worked on
-then glue the decks back down

that would be my plan anyway, modify it as need be as better information comes up

i'd guess that that kind of repair is going to need 5 - 10 sessions of an hour or 2 each and materials of around $100

problem is you MIGHT then have the main bulkhead under the front beam disintegrate or the same problem in the other hull

start looking for more hulls, if you find a reasonably priced 5.8 with good sails and trailer you might be able to part it out and cover most of your costs