Pretty straigtforward: remove the winch drum, remove the winch base (should be through-bolted through the block and
deck underneath, hopefully accesible throught the line
storage cubbys), figure out how the block is fastened to the
deck and remove those fasteners. Might be best to strip / grind the
paint off so you can see if there are fastener plugs on the top surface. It's likely bedded to the deck, possibly with something quite permanent (i.e. 3M 5200). Separating it from the deck it without damaging the deck could very well could be the hardest part of the
project. If the block is pretty rotten you can probably chisel it off in chunks, but measure it and make a drawing so it can be reproduced (less important if they're the same both sides and you're only doing one.
If you're lucky it'll be set in Dolphinite or something similar and will pop off with light prying. If not, I'd probably use a hand saw and cut it off along or just above the joint, use a hand plane and/or chisel to clean up the surface. Make sure the deck underneath is sound, or repair accordingly.
Get a block of
wood big enough to make a replacement (or find a woodworker to make it for you if you lack the skills/tools) -
teak would be best, or douglas fir if you can't afford
teak (NOT pine or hemlock! You want something
rot resistant and hard/strong). Cut to shape. If anything other than teak, I'd glass/expoxy wrap it entirely and oversize the winch mounting holes/fill them with thickened
epoxy (epoxy with milled glass fiber and microbaloons), then drill through the center so each bolt hole is sealed from the
wood.
Bed it to the deck with bedding compound of your choice and remount the winch, making sure to use
sealant around the fasteners,
paint to match... easy, right?