transom repair or replace?

[Caymanboy]

I don't think you said what year the boat is, but from looking at the stringers, it appears pre 1973.
Bringing the one stringer back certainly would help. It also appears the transom was replaced but they didn't go all the way to the corners. Have you drilled the old wood by the corners and seen if it is wet? Doesn't look like they use enough FRP buildup.
Taking those stringers up the transom, (knee, gussets) would help. You need to make sure they used enough glass to bond the plywood, etc all together.