I recently replaced all my chainplates (7, not the stem fitting); after 40 years, I thought it time, even though the old ones appeared to be in good shape. I got new ones made by Garhauer at a very reasonable price.
After pulling the chainplates - using halyards to help support the mast, 'cos I am chicken - I removed ~1/4" of balsa core in each slot, and filled the gap with thickened epoxy, so as to seal the core - now even if the chainplates leak the core will be kept dry. I then put in a small chanfer/bevel around each slot to improve the bedding efficiency (acting like a small o-ring, in theory)
The covers, as others have reported, are aluminium, and were pitted but functional. I cleaned off surface corrosion and reinstalled them, using butyl tape to bed things down.
So far no leaks. But it was surprising to me how much the hull relaxed without shrouds; it needed a couple of weeks to get the tension right!