You can get a fix on where the various bits are via things like a Thermal Imager, X-ray, or other Non-Destructive Testing (NDT). And even a small compass
might work to assist in localizing where any pieces of semi-ferrous metal are. Ditto on sounding the area with a hammer.
And given how much "fun" grinding glass inside of a boat is, getting some definitive images
of the area & hardware
would be worth it. If for no other reason than you'll save on how much glass you need to buy to re-laminate things.
Also, you can use a tape measure or indexed wire, stuck up inside of the board trunk to accurately create a sketch of the board's size & shape. Including the position of the pin. And with these, it'll aid you in better roughly discerning where the pin & hardware
are located inside of the boat.
Just index you measurements off of easily reproducable witness locations inside of the boat. Or with reference to them anyway. So that you can find the pin's location.
As to the board's bushing for the pin. I can't say what it would be made of. Though depending on the weight of the board, you might look into replacing it with something composite. That way it would be easier to bond in place, & immune to corrosion
. But if the board is heavy, read ballasted, ask a pro or three before bonding in a G10 tube instead of brass. Ditto on using similar materials in the sides of the trunk, & for the pin.
Also, much of the board's hardware, & pin may be non-ferrous. Which is another reason to look into NDT. And odds are much of it is metal, in order to handle the frictional wear from being cycled continuously when the boat's in the water
due to her motion.
PS: Try contacting a yard or three which does a lot of work on CB boats, or that builds them. And you may wish to inquire into contracting with them, or someone with such expertise in order to assist you in doing this job properly. Especially given that this is a critical component, which must be fixed correctly.