Probably cut off the transom and put on a new one.
The old fibreglass can be pulled and sanded off until you get to good
plywood.

You can buy precut gunwale material, or could a number of years ago,
wood or metal. Would be easy to come up with a list of materials and
cost out the job before you decide.

I've found books and videos on fibreglass boat repair from the public
library helpful.
Another good source is library books on auto body repair. They cover
fibreglass technique for beginners and emphasize getting a nice finish.

Looks like eminantly doable and inexpensive job for a first time
repair. Good experience.