If your existing inserts are intact, you could use them to make molds to lay up new fiberglass inserts.
To do so, you would need to thoroughly clean them and then wax the inside (the finished side) with mold release wax. They you lay up several layers of fiberglass. Once cured, what you layed up should pop out of the existing insert and you have your mold. Then you repair any imperfections in the mold w/ fairing putty, sand smooth, wash off and then wax with mold release wax. Lastly, you apply your gelcoat first, and then layup several layers of alternating 3/4 oz. mat and 6 to 12 oz. cloth. When that cures, pop it off your mold, trim the edges and you're done. Piece of cake...
If what I am calling the inserts are correct, inside of the fish boxes or storage compartments on each side of the cut transom. One over the oil tank, the other over the batteries. The boxes are very thin and are cracked on one corner not stong enough to hold any weight. Are new ones avialable from the factory? Randy