Chainplate replacement - Rotten and water soaked Knees under my chainplates

[alaskanviking]

Been Hard at work removing two of my chainplates in a Passport 42 cutter. worst thing
i have had to do to my boat yet. old chainplates are fiberglassed in
place in a kinda H shape. had to cut out the lovely teak backs, of the salon cabinets to access them. two plates on starboard were right at cabinet bulkheads. very depressing. used a Rigid Multitool to cut out the fiberglass covering. found that you
must use the metal and wood saws, as the wood only blades turn into
scraper blades far to quickly. while cutting out the chainplates i hit
3 pockets of water on the starboard plate, and 5 on the port. nasty
black ooze drained out. when i finally got the plates off the port
side the knee was all black on the bottom. tried to drill drainage
holes but the wood was way to spongy or rotten. even the wood that
initially looked ok on both sides was spongy, i could almost push the
drill bit in without pulling the trigger. i then cut one of the
fiberglass sides of the knee on both sides and they pretty much just
fell out. obviously these were not installed correctly and
fiberglassed on every side, one of the knees did not even go all the
way to the deck and had a gap of about 2". the wood looks like fir,
but its so bad i cant be sure.
its a simple job to replace them, but i need to know what wood i can
use to do so. should i go get some blocks of fir, Ironwood(IPE), Mahogany, or would
Oak be a better choice? what about replacing it all with a block of
G10 Fiberglass epoxied into place? i have a decent piece of oak that
may be about the right size. not sure which way to proceed here.

Thanks

Robert