The deck re-core is easier than redoing the bottom, IMHO. We're doing both. Our core is soft from the shrouds forward to the pulpit, also the cockpit sole forward of the pedestal, and the port cockpit bench. Use a roto-zip(or similar) with an "edge" attachment or jig, or a small 4-6" bladed circular saw to do your cutting. Use a palm sander with 50grit to remove what core the putty knife won't. In a larger area you can use a grinder with 50g on a sander attachment(LIGHTLY!). Properly bedded marine grade plywood will do instead of replacing with balsa. In curved areas, use 2 thinner sheets rather than 1 thick for ease of maintaining the curvature of the deck. There are LOADS of online videos at MAS products, West System, YouTube, etc. on how to do glasswork or recore on cockpit seats and decks. For a semi step-by-step that's VERY helpful, check out "Glissando" and all their projects.
Pearson Triton #381 Glissando | Deck Recore Project