hi,
i've done the same, i used marine plywood, the specs called for something like 3/8 to 1/2 inch thick plywood. unfortunately i had all the info i got from catalina wiped out when i switched machines. I would not use starboard for the core material as NOTHING will adhere to it. if you don't want to use plywood jamestown distributors sells all type of core material in all thickness that you would want. I believe defender does as well it is just the choices are not as wide. I would point out that there is nothing wrong with plywood as long as any through holes are properly set up (i. e. enlarged holes with epoxy filling you should not have a problem with any type of water ingress. once the plywood or core material is layed up you will need to add several layers of woven roving cloth, mat combination to build the floor up to the proper height. you should still be able to contact catalina to obtain the layup schedule for the cockpit sole. the biggest problem i had was protecting the surface while i was laying up the scheduled materials. if the area to be replaced it relatively small, not the whole sole then you might be able to get away with just cutting the fiberglass and removing it and scarfing in the plywood (or what ever core) and then put the fiberglass on top. i had to replace the whole sole and didn't have that opportunity. good luck, it is a doable project but it is not an easy or fast project. if you have some way to do the work out of the weather that would be the best option...

Take and check out Nida-core.com
This would be much better to use then any type of wood between glass.

Attached is the glass and reinforcement schematic from Catalina. Pretty sure it covers your year but you can look at the dates and see. According to this the cockpit floor is 1/2" balsa with .75 Mat. Refer to Section E.2.

If you want to replicate the non-skid you can either order the pattern from a Gibco distributor or make a mold using silicone or latex molding (check a plastics or mold supplier). I cast a mold on my cockpit floor and got a good pattern.

If you have delamination you can carefully cut the floor using a fine blade (like a multi-tool) just inside the edges of the non-skid, the using some flat blades (I used aluminum bar sharpened on the end) slowly pry the skin off without breaking the glass. replace your core, add some glass and then epoxy the skin back. Fill the saw marks and paint. If you cast a mold you can use gelcoat paste to fill the cuts through the non-skid and perhaps get a decent look for the repair.

thanks to mutual fun for the basics of how to do this, haven't tried in the cockpit (thank god I don't have that problem) but it worked on the side deck.