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Mark,

What are your plans for the boat? You can restore the non-skid if you want but it will take some work. Not sure what shape the remaining gelcoat is but here are your choices,
1. Follow Rich's suggestion if you plan to paint the entire interior.
2. if the gelcoat is in good shape and restorable on the sides you can redo the non-skid. You will need to make a mold of the non-skid that is in good shape. Then cut the bad section and replace the bad foam. Replace the fiberglass matt. Follow these instructions West non-skid repair
You will need to make the mold yourself as Gibco does not make a mold that matches the non-skid on the 13 footer. smooth-on.com is one place that has the correct material, 50 hardness is what has been used before, Reoflex