I did the this job three years ago and photos are in the following link:
S/V Victoria Head Reconstruction
The topic was discussed in some length in the following link:
Chainplates, Bulkheads, and Woodwork
The job is major but not insurmountable. I do think that by the time you find any good, dry wood in the bulkhead, you end up replacing the whole thing. Another problem with "splicing" in a new section if plywood is that this is a structural area that supports tremendous loads and could tear out. Better to bite the bullet and resign yourself to the idea of some deconstruction. I'm not sure what that aluminum plate is - a backing plate added by the PO because the bulkhead is rotted? Makes me worry. After you jackhammer the old 5200 off the chainplate, route out a shallow "pool" in the deck (1.5"x1"x1/8") through which the chainplate passes. The new 4200 will sit in that pool and have a larger area to ahdere to the deck and chainplate. Shaeffer sells a nice cover to dress and protect the area (basically a flat plate with a a rectangular hole in the middle and a hole in each corner for screws). My detailed instructions from the previous link follow. Good luck, it's not THAT bad.
1. Take a deep breath, it's not as bad as it looks. I had the same situation when we bought Victoria, our 38. The whole job took about 25 hours.
2. Take pictures during deconstruction so that you know where everything goes. Carefully remove all the trim and mark where they go on the reverse side. Save the vinyl hull covering too. Remove the chainplate hardware. If the mast is still up, use a halyard as a temporary stay.
3. Remove the shelving, DO NOT rip it out. You'll use all the plywood as a template. Basically, you'll have to unscrew the mounting cleats from wherever they're attached. Note that Sabre used resorcinol to glue the cleats to the plywood, so you'll have to pry them apart. SAVE THE SCREWS
4. Once the bulkhead is exposed on both sides, you'll need to detach it from the hull. Use a heavy duty cordless drill with a dry wall routing bit (the kind used to cut out receptacle boxes) to cut away the tabbing. I used a single bit, but have a spare ready in case it breaks. This is heavy work so wear hearing protection and a dust mask. Lay the drill parallel to the hull and slip the bit into the tabbing and work from the deck down along the hull. It's slow going, but will work.
5. Extract the bulkhead in as complete a piece as possible. Save all large pieces of plywood.
6. I used Lowe's exterior ply. NOT the nasty underlayment, but the 6 ply stuff. You don't have to worry too much about using marine plywood since YOU WILL keep your chainplates well sealed after this job. You'll probably need teak marine plywood because the bulkhead is exposed (min was in the head and formica covered). There are many sources.
7. Lay the nasty templates on the new plywood and carefully trace the outline. Cut with a jig saw to the line and make clean cuts. The bulkhead side doesn't have to be too fair, but definitely plane the exposed edges smooth. Use a plane and not a file - you'll get a much better job.
8. The hull edge of the bulkhead doesn't have to fit the contour exactly, but should be within a 1/8"-1/4". Tabbing will fix the rest.
9. Lay protective plastic and cardboard everywhere. I should have written this at the beginning, but you'll need it now.
10. Buy West Epoxy and fast set hardener. Buy heavy woven roving (WM Model #: 154013), NOT mat. Cut a continuous strip as long as the hull to bulkhead edge and about 8" wide. You need at least 4" lap on the hull and bulkhead on BOTH sides
10a. Attach the bulkhead to the deck and/or any cabinetry with screws to hold it in place during tabbing.
10b. I should have done this step, but didn't..... Mix West collidial silca filler with epoxy to make a paste and use as a temporary glue to attach the bulkhead to the hull along the hull edge of the bulkhead. Note that you MUST remove the squeeze-out. There can be NO lumps that impeded the roving layup. This is easily accomplished by taking a putty knife and pulling it along the bulkhead/hull edge, removing the excess putty. One pass on each side should do it.
11. Completely saturate the roving while laid flat on the plastic covering on the floor. Use gloves. Pick up an end and quickly transfer to the bulkhead/hull. It will be a dripping mess, so practice first while dry. Roll out with a resin roller. Make sure that there are NO bubbles.
12. Wipe up any excess dripping resin. DO NOT leave the job until the resin sets up. It could be a mess otherwise. Do not overwork the roll out as the resin sets, or it will begin to pull up. Basically just roll it out when you first lay it.
13. Reattach the shelving, etc. You'll probably want to pre-finish the wood with your choice of finish (I used satin urethane). Reattach trim using the legend that you wrote on each piece.
14. Reattach the hardware, redrilling the chainplate holes and SEALING the deck where the chainplates pass through. 3M 4200 works well