I got stringer problems!

:o :o :o :o :o :o :o :o :o :o :o :o :o :o :o :o :o :o :o :o :o

The previous owner should go to jail for V20 abuse.

But you are still lucky the transom is sound, and you caught the stringer problem before it spread too far to get at it to repair. I think I would have taken the glass off the tops of the stringers, removed the rot, then glassed the good ends of the stringers with a layer of matt, filled the void with seacast, and then reglassed the top of the stringers with matt and woven roven.