Yes for sure agree with Woko who is always spot on.
That photo is not good obviously.........it is very zoomed in please can you post a few more photos progressively zooming out so the extent of that damage can be seen around the chain plates.
Many small yachts have deck lift around the chain plates potentially due to unacceptable strength initial design of the chain plates and ply bulkhead they are bolted to, or water ingress from the deck down under the chain plates so as to rot and weaken the ply, or corroded chain plate bolts, or a combination of all 3, or other.
The S22 have a good class association here and would suggest you contact them - one of their members will surely be able to diagnose the root cause source or sources of your deck issues and advise a solution. See also facebook page;
www.spacesailer22.org.au/association/index.htmlwww.facebook.com/Space-Sailer-22-318676201822023/They should also be able to answer your question regarding if the the chain plates can be bought off the shelf, or if they have to be custom made. I would doubt if you could buy them off the shelf now but hopefully the class assoc has design drawings where you can have a suitable stainless fabricator - probably in the Henderson area - make them up.
The chain plate design does not look too flash sorry to say - very unusual to have the cap shrouds and diagonals having separate welded fittings in a 22footer. Maybe they go down into stainless flat bar straps each side of the main bulkhead, securely bolted with a suitable array of bolts in shear. The ideal is to have a single suitably thick and wide and long stainless flat bar for both shrouds per side, securely bolted to the main bulkhead - no welds. Inboard hole for the diagonal, outboard hole for the cap. Possibly the class association can advise if such a design improvement has been instigated. It would be very easily implemented.