Traditionally, the rudder would be solid mahogany. You are probably ok using plywood for a rudder blade, but perhaps not. Keep in mind that since plywood is usually oriented with a 90º offset for each ply, the finished plywood board is only about 1/2 as strong to a lateral force. (This orientation makes plywood dimensionally stable.) 1/2 of the plies would run up and down, and be fairly strong to a side load, but 1/2 of the plies would be 90º offset, and would not be strong on a side load at all, and would in fact be very weak. Imagine how hard it is to break a paint stirrer longitudinally, but how easy it is to break across the width. The centerboard for my GP14, which is a similar design vintage to your Wayfarer, was made with laminated solid mahogany strips. The grain of all pieces is oriented longitudinally, but each piece was likely end-for-ended, so that the board will resist warping, but is still considerably stronger to lateral force than plywood. Imagine trying to right the boat from a capsize, and the plywood board breaks off because it is only 1/2 as strong. A rudder foil is probably ok, being smaller size, and probably not seeing as much lateral lifting force as a centerboard, but it too could break. My boat came to me with a plywood rudder, but a second boat I purchased had the solid mahogany rudder. I sold the second with a newly fabricated okoume BS1088 board, and kept the solid one for my 1st boat being restored
I suppose you could cut your new rudder blade with both plies 45º to the length of the blade, but I still think it would not be as strong as a solid board.
Seats I have seen on Wayfarers (and my GP14) are solid mahogany slats. You can see that very nicely on this pic of a Wayfarer:
The floorboards in my GP14 are cedar slats screwed into the mahogany frames. (Oh, right, it is a wood GP14.
) I have seen plenty of boats with plywood floorboards, usually with riser pieces fastened underneath every 8" or 12" or so to lift the boards up off the floor of the boat. You could consider painting with non-skid paint, or maybe try it old skool and varnish. While still wet, sprinkle granulated sugar on the varnish. When dry, wash the sugar away with water, leaving a stippled non-skid varnish finish.