Just recored a huge section of deck on my boat over the winter. If there is a structural rib that the stay bolts to under the deck, then I'd say go from the top. Cut out a section of decking twice as big as you think you need, remover the outer skin and then the bad core. Recore with balsa or thickened epoxy, then epoxy the skin back over and redrill/cut out your slot for the tang. Fair and paint/non-skid.
These pics are from going from inside, but my boat is pretty stripped out of any interior contours.