Let's say you intend to
refit with exterior chainplates.
Exterior chainplates usually are through-bolted through the hull, with a backing plate on the
interior of the hull, so the bolt tension clamps the chainplates to the hull, right?
If you wanted to maintain your
current rig geometry, you could use your
current embedded in GRP plates as the
backing plates.
But you would need to drill through both hull + existing chainplates to do so.
Drilling through the hull + existing chainplates may take almost as much effort (in terms of dismantling interior furniture etc) as removing those embedded chainplates.
And you would also need:
(1) to check that the existing chainplates are solid enough to act as
backing plates (that is probably the case: the corrosion is likely restricted to being around the exit of the plates through deck); and
(2) make up a template of the hull curve so you can have the plates bent to fit the hull curve (otherwise when you clamp an exterior plate to the backing plate, you will bend the hull to suit the exterior plate).
You would need both a template of the hull curve (at each of your chainplate stations, starboard and port), plus you would need to
work out the dimensions of your plates. You have the thickness and probably the width of each plate, because you've plate exposed above deck. With careful use of a quality metal detector, you might be able to
work out the length of each plate.
As for making new chainplates, in SE Qld you need jolly good luck to find either forged
bronze plate or G5 titanium and someone to work it. I'd suggest staying away from cast bronze plate unless you know bronze and the caster well.
316 stainless is
cheap. It's a compromise: subject to crevice corrosion and stress corrosion.
A little more expensive is 2205 duplex stainless. Almost immune to crevice corrosion and if in the same scantling as your 316 stainless original plates, relatively immune to stress crack corrosion (scc) too.
For bolts, you could buy future
insurance and get the bolts and nuts carved from 2507 super duplex. It's even stronger than 2205 duplex and even more immune to crevice corrosion and scc.
The downturn in the mining and gas pipe industry in Qld means you can find suppliers of 2205 duplex and people to machine it for you. As Stumble noted, polishing 2205 duplex to No. 8 mirror would double the cost of the plates. Electropolishing to slightly less than No. 8 mirror is more economical.
If you wanted to go down either the El Cheapo more 316 ss
route, or the long life 2205 duplex route, I'd suggest you go talk to Mykel at Stirling
Stainless Steel at 51 Musgrave Road, Coopers Plains. Or look for other contact details at stirlingsaustralia.com.au You'd need to make drawings of exactly what you want, including the bolt holes. Stirlings would then laser cut the
steel plate (so if you wanted to use carriage bolts, you could get the square holes cut to the right size and without stress to the metal).
Mykel could also point you to expert fabricators and electropolishers.
If you want 2205 bent to a precise shape based on a template, I'd point you to Karl at NRG Piping at 55 Christensen Rd, Stapylton (it's a small industrial park. NRG Piping is at Lot 6).