It's easy enough to take apart, so here are a few pics.
How the components relate in situ.
The valve components as they fit together inside the cylinder.
Not sure what the original valve seat material is, but a urethane replacement is the obvious way to go. It would be a simple enough rebuild, except that I don't have a lathe or mill or anything like that (hence my attempt at repair using sandpaper and a file!)
Ken Walker