RE: Cost of Transom Rebuild
What about some aluminum bracing through bolted to the inside of the transom? For instance, if it were possible to access the entire transom from the inside, it might be acceptable to span the width of the transom with heavy gauge aluminum channel, through bolted to the transom.
If your problem is the lack of rigidity of the Seacast product (and not addressing core rot), then maybe bracing or stiffening of the transom is all that is needed.
By the way, if Seacast relies on the fiberglass for it's strength and rigidity, then what purpose does it serve? Afterall, the entire purpose of the coring material that Seacast is meant to replace, is to provide the strength and rigidity.