I'd go solid not wood core of any kind under the mast. Stillraining is right - the only reason builders use plywood is that it is cheaper and is also faster than multiple layers of glass. I'd use epoxy (West as I know it well) and rather than mat and cloth or roving I'd use 1708 biaxial - stitched as opposed to woven roving with mat already attached to one side. It's easier to use and fully compatible with epoxy - some mat isn't epoxy compatible. If your deck is already needing repair going from the top is always the best way - it's easier and the job will turn out better. I'd paint after instead of gelcoat. For a good rundown of this see Tim Lackey's site - Northern Yacht Restoration | Tim Lackey:* One Man, One Boat at a Time
. He recores decks on a regular basis in his restoration business and documents everything with lots of pics. Just click on one of his projects and follow it along.
Hope this helps.