What is this called and where do I get one?

So I'm replacing the Transom of my old Montgomery Ward Sea-King Boat. The boat has taken some tumbles over the year and isn't exactly symmetrically shaped anymore but I'm hoping to at least get the corners back in shape. One of these pieces pictured is broken, and I am trying to find a replacement. I'm not afraid to drill some new holes if I can't find the exact piece. It looks like cast aluminum, I'd be afraid to try to have the other piece welded, with all the imperfections in cast aluminum I don't want it going POP!