The Day Sailer decks were not cored, and be very careful if you decide to replace the deck with anything different from the original since Class Rules prohibit any changes to the deck. I'm not even sure that my boat is "class-legal" after I added an afterdeck (similar to that on the DS I) to my DS II. Luckily it would be easily removed (well, not "instantly", but no major surgery to the boat would be needed) if I sold the boat to a racer or decided to enter a DSA Class regatta myself.
Other than more classic asthetics.....why would you want to replace the fiberglass deck with wood? (Admittedly, my afterdeck is fiberglass covered plywood.)
However, if you are actually refering to the wooden floorboards in the cockpit of an older DS I, I supose you could use some kind of foam-core, like Divynicell (sp?) or such and cover it with a couple of layers of fiberglass cloth. Marine Plywood coated with Epoxy might be a good substitute for the original narrow planks/strips/grating. You could even recreate those thin strips in plywood as long as you seal them well with epoxy and then paint or varnish to protect against UV. Be sure to avoid letting them sit too long in bilge water though...... I've made a pair of floorboards to cover the shallow well at the aft end of my cockpit out of epoxy-coated exterior plywood. So far it is rare to get more than 2-3 seasons out of a set before water finds it's way into the lamination and they start to come apart. Would really like to make a set from "Starboard" or such but $$$ is short in my boating budget and plywood seems more economical (yet, considering the number of times I've rebuilt my plywood floorboards............maybe synthetic would ultimately cost less?)
I've included 2 pics, one shows the floorboard set fro man old DS I and the other shows my afterdeck. I'll try to post a picture or two of my aft floorboards later.