Thats the actual valve, and doesn't look that worn. Have you got a better shot so we can see where the wear is? At a guess Im betting your missing the small spacer that goes on the end that inserts into the hole on the inside the headers,
part 13 (90387106u7)And regarding the bearings in the end-plate and the headers these are phosphor-bronze, I very much dought you can put normal ball based bearings in there due to the heat 200-400c. For a start the great in them would be burnt away, then there the issue of different expansion level of the balls and the shells.
On while we are on the subject, I would strong suggest replacing the cover and plate bolts (number 7, 18) with either Stainless or titainium (I went for Ti, and I've had them for years just moving from one bike to the next)