Call MC Direct... as Jersey says... the Tower Maker is in the Maryville, Vonore, Loudon area along with all the other suppliers that MC uses to out fit a boat. Similiar issue just came up with the Bimini I boat off here... dealer said MC "doesnt carry or have" the mounts any longer. A quick call to MC, I found out who at the time the supplier was, and I now have mounts coming.
Welding... would/will be your only solution next. Having a Shop, using the Cad drawings... programming, setup, and machine work that one piece would cost ya BIG unless you know someone at the shop who wants to do just for the fun of it.