Source for Transom Shower Replacement?

I would check Flounder Pounder, or replacementboatparts.com

 

What are your actual dimensions?

 

Since that piece appears to be fiberglass, I'm going to venture a guess that it was made in-house at Sea Ray. Which means it's a custom piece and (aside from being nearly 30 years old) would explain why you can't find an exact replacement. You could repair the fiberglass piece, though, and then either look into rebuilding the shower stems or just replace the knob and wand pieces.

Another option, that might make things easier, is to first make a cover plate out of HDPE (white or maybe offwhite) and then cut a whole in that to allow the use of a smaller fixture. The HDPE would end up just looking like a trim piece around the edge.