I agree with Chuck. The balsa core causes the deck to have structure much like an I-beam. Without the interior fiberglass, you've got nothing.
What has been done with success, but more work initially, is to save the interior fiberglass as a panel. Carefully cut around the area you wish to cut out, and very carefully remove the fiberglass by cutting the core out. Then you can grind off any remaining core material from your panel. Then you can replace the core. To reinstall the panel, wet out the balsa core, the apply thickened epoxy to the panel. Prop the panel in place making sure it has good adhesion all the way across. Then fix the cut lines and you're done.
Or you can do what I did. Hold a 50 x 60 inch piece of dripping wet 18-2 biax over your head for about an hour.
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John USYacht 27 "Cora Lee"