For cutting the existing wood out of the floor, look into getting an oscillating tool and a saw blade attachment.? That's a great tool for working in confined spaces, and not too expensive either:



I know Lowes will cut plywood for you.? Probably Home Depot too. They should be able to cut the main rectangle of the size you need.? But then you'll probably have to cut anything else.

The tricky part about doing a partial floor repair is getting the new wood under the rear and side wall.? You'll probably have to cut the hole bigger than the piece of replacement wood, then drop the wood in and slide it over.? Then put a small strip of wood alongside the bigger piece to fill the gap, and then put a sandwich layer on top to hold things in place.

When you get in there you might also find rot in the wall.? That would complicate the repair.