That's a great looking boat, it will be worth putting money into it to fix typical old boat problems. Now what I'd do:
remove the hatch over the gas tank
see how far that rot goes
if the plywood is just rotted in one spot, but solid below it, you can cut out the bad section and scarf in a new section. You have to be very careful though because the tank is right there, and the fuel lines are too. If its all bad, then its a bigger job. I'd then have someone help you drain and remove the fuel tank and fix any rot you find in that area. And please GET RID OF THE CARPET, its what causes rot in the first place. Rip it all out, and have the deck re-glassed and gelcoated in non skid gelcoat.
take a look at this website:
www.rotdoc.comif you use the CPES, make sure to wear a real fume respirator, gloves, googles etc.
I fixed a spot like that on the front bulkhead of the tank compartment about 5 years ago, cut out all the bad wood, treated with CPES, cut a patch panel and used their epoxy filler to fill in the gaps, glassed with epoxy came out great. CPES will kill the rot spores and small insects lol....