Yes, transom can last forever, make pattern, 3 layers of ext. Plywood 3/4, glue together(gorilla glue, the foaming type) screw together, let set(week), scape off excess, mix fiberglass resin with hardner then cut 1/3rd with acetone then start coating with roller multiple coats, it soaks in good, as I said , multiple coats, quickly before it gels up, let dry and get hard. Waterproof forever, never rot. I've built many, Never had one go bad in 40yrs.If it were mine ... I'd make 'smart' cuts as needed to then allow it to be bolted back together the strongest that I could. Those supports from the floor angled up to the transom are called the 'knees' and I'd keep them affixed to the floor, as with that side flange, you can then bolt them back in place up top to the new transom.
Simple way, once you get the 'top cap' out of the way:
https://www.tinboats.net/threads/new-transom-for-tiny-tinny.43039/
Now this post here may be overkill, but this is how I do replacement transoms and maybe it will give you some ideas.
https://www.tinboats.net/threads/new-transom-–-1976-starcraft-‘holiday-15’-model.47405/
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