do you think epoxying some marine plywood to the inside over a large area (3'x3') would do it?
That should help. It would be best if your plywood extended
the entire height of the transom, and was tabbed with fiberglass
at top and bottom. That will transfer some of the loading
to the hull and deck structure.
That is quite a large motor for a ODay 27,
and I think you want to eliminate any flex that
might eventually fatigue the laminate.