Hi BOBG, Welcome! The issue of failing teak decks is very common and well documented in terms of replacement on older trawlers.
Keep in mind the failure typically extends past the failing teak layer as the cracks, checks, missing bungs, failing caulking and bedding will allow water to penetrate the deck through the screws used 40 years ago to attach the teak to the deck. Water then gets into the deck coring (which is usually luan squares or plywood) causing rot. This means a boat with failing teak (waterproofing layer) usually has soggy decks as well.
So, you may actually have two projects to consider:
First: Remove the teak and outer fiberglass skin then the soggy core sections and replace using modern materials and a shiny new fiberglass skin. This gets you back to structurally sound and waterproof.
Second: Replace the teak the old school way with individual teak boards cut to fit in place (and glued rather than screwed) or replace with pre-fabricated teak "mats" installed in large sections based on templates you make and send off or replace with fake teak (same templating process) or do what most people do which is to not replace the teak but instead use a non-skid like Awlgrip (the "glass over" approach).
Some people might argue that improves the vessel in terms of modernization and proof that the decks are repaired.
Anything can be done with enough time and money but the cost of repairs is hard to estimate without getting into the project. Most shipwrights around here (PNW) are in the $60 - $90 per hour range which adds up fast. Fake teak costs about $20 - $30/sq ft assuming a DIY installation, more for templated mats and more for professional installation.
There are a bunch of great videos about this topic on YouTube, but BoatWorks Today in particular address both deck core repairs as well as a complete teak replacement on a sailboat.
https://www.boatworkstoday.com/videos/
The rub is, while the repairs can be accomplished DIY if you have the skills at a significantly reduced the cost (the expense or materials and the value of your time) but a professional replacement will likely exceed the value of the vessel.