Keep it original?

[FLexpat]

The original deck was part balsa and part plywood cut into little squares; maybe 50/50. Balsa is about 5 lbs/ft3 and general plywood is about 40 lb/ft3.
Okume ply is about 28 lb/ft3. Stuff like corecell is 5 lb/ft3 and coosa bluewater 26 is 26 lb/ft3 (you can get 15 too). With 1/2", you will probably use about 2 ft3 for the whole deck of a 23CC so the weight difference is not huge. The big question is how well you will be able to keep water out of the core. Think about putting small blocks of solid glass where you know you will have penetrations- makes it easier later but can be a giant pain to figure out the right spots. I suggest you cut the core to fit just inside the lips/cleats and bevel the edges at 45* from the bottom so that the top glass and bottom glass have a couple of inches to stick together (seal) around the edges which rest on top of the lips/cleats. If you just lay glass on both sides and it does not come together n seal properly on the edges you will eventually get a failure there and probably get to redo it in a few years. Lay it up upside down and trim it to fit. If you use epoxy for your layup then use a thick epoxy to bed it - not sure if Arjay makes one but their VE 7121
works much better at bonding VE or Poly.

You are right about the 23 not needing the deck raised unless you are adding tons of weight (twins on a heavy, non flotation bracket) or loading it real heavy; however the 20CC does need the decks raised for a bracket or a heavy motor.