I would suggest using glass and resin after you have cut out as much of the bad stuff as you can reach through the deck cut. The glass resin penetrates and seals the core where you could not reach. With a resin paste, you may not get this sealing action and in fact may leave hollows. When I did mine, I used fiberglass mat, not the wovoen glass cloth, just to bulk up the gap (alongside the starboard aft window). Then I covered it with the piece I had cut out of the deck and leveled it so that, when cured, the deck would again appear intact. I am assuming that the resin will not run straight through into the cabin, in which case you need tape and a piece of plastic to block the leak.