The issue with this generation of E-boats is that the chainplates were custom-built structures laminated into the hull. Replacing with "like" ones requires custom work and demolishing and rebuilding that section of the hull. Although I think the site has at least one story of an owner who did just that, replacing them with conventional chainplates is generally regarded as a far cheaper, simpler, and (because they can be inspected) safer approach.
Note that the internal frp molding will never need to be replaced unless the hull suffered catastrophic damage to the side. The aluminum bar can, I believe, be removed when the ss rod is fully unscrewed. I might have guessed wrong, but that is what it looks like. They did put some glass mat over the inside end (backside) of each bar, but I believe that I could cut that off with a small die grinder.
I have read of owners (of other makes of boat besides Ericson, I should note) adding external new ss chain plates (shrouds) when they no longer trusted the glassed in tangs on the inside of the hull. For instance, "going external" was the method chosen by an owner of an E-35-2, reported somewhere on this site.
By the 80's, EY went with external back stay plates on most of their boats, including ours.
(if I ever find a crack in ours, I would get a quote on a TI replacement. As for cost, I would first get firm quotes in hand based on my drawing.
If going with SS again, I know of a local machinist that could help me spec out a new one, or maybe fab. it up himself.