Check out local welders ...re new sheets of SS. got to be some decent marine guys near Cape Canaveral. we did that with a local boat down here ... and while the welder was more into biminis and SS tops for fishing boats, he didn't blink when he saw the original chain plates .... [not from a j30 repair]...
Welders can use the originals for the top pattern. and wing it on the belowdeck. Suggest you go one thickness up. where were the cracks ... visible or betweendeck?
not impressed with the aluminium suggestion though. and don't remember seeing a plastic backing plate in use.
if the shrouds are original, you need to really check to see if there are any broken strands mostly at the swages ... generally we change shrouds every 20 years ... but with the cracked chainplate they may have gone though a "shock".
Hall used a beefier turnbuckle last time I had the shrouds done on 505.
