Provide bulkhead support
I had a similar problem on my 25, the cockpit sole was cracked and the core had rotted. On looking underneath I realized nothing supported the entire cockpit area. What I did was this, first I epoxied a piece of 1" marine plywood to the underside of the sole, I held it in place with stainless screws every 8" on either side. Next I made a support bulkhead and attached it to the plywood reinforcement at the center of the piece and down to the hull. I tabbed the bulkhead to the hull and the sole support. Now that the sole was fully firm and supported I cut out the damaged core, did an epoxy fill, and reglassed the cockpit sole area. As I was repainting the entire boat anyway, it all blended in.