If I were looking at updating the valve train I'd purchase (used?) IN and EX XR valves from Darcy (darcyracing.com) or a flat tracker who runs XRs and copy the shapes on either a new set of valves from, e.g. Kibblewhite, who will make small batch or one off valves from their inventory of blanks, or use a lathe to cut down suitable valves. If the engine isn't a max effort with severe cam ramps, high spring pressures and extended periods at 8000+ rpm there shouldn't be any breakage problems and you'd have valves with shaping that reflects current flow thinking. Harry didn't do any further XS engine development after the late '80s, so I have to assume the ongoing effort to keep the XRs competitive has resulted in some flow advancements since then.