Pretty sure they will be standard imperial thread bolts - you could purchase a small selection from the hardware store, take them to the boat and see if they fit - think they where coarse thread but don’t quote me on that! If you where to get 7/16, 1/2, 9/16 and 5/8 bolts in coarse and fine thread then just try them in the openings it should identify your diameter pretty quick. (I’m thinking they where about 1/2” but not entirely sure.)
Once you’ve identified the diameter you can stick a piece of dowel, branch, skewer etc into the opening to get an idea of length you will need. Subtract a bit from the dimension you get from this so you don’t bottom out the bolt.
Once you have the size you will want a fender washer and possibly a lock washer to go on the bolts. Original parts are all stainless. (Add these thicknesses back to the above measurement. )
