So, the center plate is a problem.
Clearly the best thing to do is to remove it and cut another. However, no-one seems to know how to do this. I have consulted various oracles in England and they have been strangely evasive. The problem lies with the location of the center plate pivot bolt. The plate has to swing up and down around a pivot, which is, it seems, hidden behind layers of fibre glass.
You take off the access plate only to find fibre glass.
We take off the wooden center plate surround on the keel. This is not easy.
It is a fine piece of wood. But infested.
Taking it off does not really advance me in the search for center board bolt but it does give an injection of romanticism.
In all my readings of voyages to the tropics, I have always been struck by the danger of the Teredo worm. Said worm burrowed into the wood of boats and basically ate them. This was a major problem. https://en.wikipedia.org/wiki/Teredo_navalis
Check out my very own Teredo worm! How romantic is that?
Anyway having totally exposed the centre plate, I decide that we will repair rather than replace.
Voyage of discovery.
The Quiet Ripple Defines The Pond 
Those Teredos having been working hard. Will you be constructing a new centre plate surround? Creosote seems to only answer.
Repairing the centre plate sounds good but if you can’t remove it completely doesn’t mean that you’re relying on the strength of the parts you can’t get at, especially that pesky pivot bolt? Is it not possible to driill, abrade or even burn the fibreglass out?
Worm damage not that bad. I will treat with wood stuff and then anti fouling. I too worry that the bolt might be past it and the plate will drop out miles at sea in a typhoon etc.