You can try replacing the flexible supply line first—I’ve had those become obstructed with god knows what I’m the past, making the toilet really slow to fill. But as others have said, don’t try to just replace the valve, you’ll need to replace some lengths of pipe as well. Looks like it may be galvanized, in which case you can screw some new pieces together or replace with copper or sharkbite. As for the floor, don’t worry too much about how to clean it, if it were me I’d just live with it until it’s time to replace. This bathroom likely needs to be redone!