Without knowing the exact situation, a very general method would be as follows.
Cut a hole in the deck so that you can reach inside and repair the trailing edge of the case from the inside of the hull. Then replace the cutout in the deck (you should keep the bit you cut-out).
There are a lot of alternative ways to do this depending on the original construction technique and the extent of damage.
Really need to know:
- Extent of damage to the hull
- Extent of damage to trailing edge of the case
- General hull laminate details (cored, single skin? I'm assuming it is a composite construction)
- General case laminate details
Do the repair properly first time, don't jam a tube of sikaflex into the damaged area, you'll just have to remove it later before repairing again.