I'm assuming you are referring to the foot traffic "deck" odf wood, and not the foredecking of fiberglass. Yes, that was made of plywood, it was light in weight, it took the full brunt of the weather on top and had moisture from the bottom side as well. The good news is, it is pretty straight forward of a replacement. I used white oak, similar to what Glenn Feilhauer is using on his boat, knowing white oak was used for the steam bent ribs on the wood Sea Skiff boats, and with a good bilge coating it should last a very long time.
If you use a good marine plywood you won't have to worry about it in this lifetime, is my hunch. If you use regular exterior grade CDX plywood, all I can say is "good luck" because that stuff will not (repeat will NOT) withstand the marine rigors, and I figure I'm preaching to the choir with you (and just about everyone else here on The Forum as well).
There are so few of these boats left, most have not survived in garages and boat houses, but have been run until they encountered serious mechanical wear, which is natural for any machinery. Once worn, however, the then "old boat" would require a fair amount of work and perhaps cash too. They were then set aside for "later". Only problem......"later" never came for most of them, until people like you, me, Petter, Dave, and Eric got the boat and took on the work.
As with Eric (Jensen), his Corsair (23 Lancer) had wood decks. They had to be replaced. My 23 Lancer is a newer model and it has a molded in fiberglass deck, so that issue has been dealt with on the evolved model line. Most all of the older Corsair boats had a pretty royal hull, but used a lot of wood too. The good news is, the wood part is the easy part, as the fiberglass hull structure is generally going to be good enough to get back into service. As with my Skiff, however, I did encounter a structural piece that was torn loose, which I repaired with fiberglass roving and epoxy. It is now probably stronger than new. Hope your Corsair project is coming along.