That's going to be a tough one to fix because of the mast step area. It is fiberglass and with time, patience, and money it can be fixed, maybe even improved.
If you are adventurous you might want to redesign the mast step. Some kind of a box or tube that is glassed to the hull. This would transmit the loads directly to the hull and help prevent this from happening again. The deck would then be easier to layup and it would help stabilize the new mast step.
Is there easy access to the area under the deck, through a small cuddy cabin perhaps? If so, it may be easier to turn the boat over so that you can get gravity working for you, at least for the first couple of layers of glass. Once there is a base to work from, flip the boat back over and lay up the finishing layers.