Core rot in the decks of old boats is the unfortunate reality, as prior owners (and even the factory) seldom take the time to drill-fill-drill (smaller) all the holes in the deck for fittings. And so when the sealant fails (and it will eventually) then the water seepage into the core begins. One of my prior owners installed a bimini with tracks, by just screwing into the deck. What were they thinking ! And so , of course there is some small areas of rot. So far I have lived with it as it is on the side.
There are tons of projects on YouTube of core replacement from small projects just around a stanchion, to whole deck replacements. It is hard to beat the extensive DIY projects of Mads Dahkle and his Warrior 38 "Athena" on youtube channel "Sail Life". He is fearless in tackling any project. In his series, he replaces the entire top deck. Whether you do a little bit, or a more extensive project, I cannot imagine doing it from the bottom (with dripping epoxy everywhere). Cut out as much top as needed, recore, refiberglass, and then top it off with KiwiGrip nonskid solution. It isn't going to match the other parts of the deck, but hey - it is a good old boat, and it will be secure for another 30 years. If your non-skid is grey and KiwiGrip is also grey then not too bad a coordination. I did my lazarette hatch as water migrated into the core all the way from the edge hinge area. (again factory sloppiness).
Google "youtube Sail Life core replacement" to get started on about 10 or so episodes completely documenting the process. "Sail LIfe"
"SailLife" channel and Andy's "BoatWorks Today" are the two best DIY boat repair channels out there.