The "advanced" 1953 Chevy Corvette had a fiberglass body—which actually followed fiberglass construction in boats.
By the 1980s, boat manufacturers had figured out how much fiberglass to leave off.
There's a good chance the wood that makes up the transom has been compromised. I'd move the boat about three feet back on the trailer, take a reciprocating saw to the damaged area, remove everything that looks like damaged wood, and build the area back up with lots of fiberglass mat and cloth. I'd cut away about three times the area that appears cracked.
What you have "repaired" there appears to be
Marine-Tex, a substitute for gelcoat, used to make cosmetic repairs. Like gelcoat, there's very little strength to it.
Use
epoxy resin, as it is odorless and
very strong.
Sand smooth and fair, and use paint to match. Don't bother with gelcoat, as it's expensive, and too difficult to match color—even with white.
You'll have spent about $300 to save this old boat.
As for sideway pictures, be sure you've hit "save" when working with photos.