I removed the chain plates earlier this week and had new ones made up by Grant's Marine Service. Grant re-purposed the old chain plates into backing plates as the plastic ones showed signs of crushing. Total parts costs were $222 plus new bolts ($60 ) and new 1/2" pins for the turnbuckles ($30).

Here are the new and re-purposed parts:

Thankfully the balsa core was dry at the chain plate holes. They were sealed with penetrating epoxy. The bulkhead was also in good shape with no rot. I'll be sealing the chain plate holes with "thru the roof" caulking from Ace Hardware.

I also replaced all the cooling system hose clamps with the closed variety. After adjusting the valves I have a slow oil leak from the valve cover. Will try that again.