I replaced about 1/2 of the rotten bulkhead of my Catalina 22 by cutting out the rotten part and using it as a template for a marine plywood replacement.
I doweled and Gorilla glued the new wood to the old wood, cleaned it up and it seemed to work well. I'm confident that the fix was at least as strong as the original.
I was able to remove the bulkhead for this, but imagine that you could do the same thing while the bulkhead is in place.
I would have used teak-faced marine plywood, but couldn't find any where I was (Tulsa) at the time. So, I laminated white Formica on top of the new wood to hide the joint. It looked quite nice, frankly.
I think this was a good fix. Others, with more experience in this matter are likely to chime in with better ways to do what I did.
Frankly, since you know boats, and as long as you are being careful, I wouldn't let this problem stop you from buying the boat.
1987 CS 36 Merlin "Kyrie"
"They drove a dump truck full of money up to my house. I'm not made of stone!" -Krusty the Clown