Help with transom repair

by Jared
(York County, VA)

Recently acquired a mostly finished wood boat.

Builder apparently became ill before his wife then donated to local boat museum where it sat partially covered under a tarp for 5 years or more.

Has never been in water.

Stern was uncovered and looks like water was able to seep into transom.

Bottom was glassed but you can see horizontal transom boards pulled away from gunnels at the stern.

I removed some of the horizontal boards to reveal a much thicker vertical board where there is a small amount of rot in the center.

Do I rebuild the entire thing or can I cut out the affected boards and try to replace with a similar wood?

Obviously an entire rebuild would be more complete, but the joinery at the corners looks seriously daunting to me a novice.