Yeah that one. Uk forums are dead now for those bikes unfortunately.
Is one of the exhaust valves seems to keep closing up its gap every 6 to 8 k. Not massive but it is on the move. Cyl heads are cheap though so will just swap it out when it inevitable eats through the seat
BMW/aprilia so the rotax 4/5 valve a cylinder?
There are kits but never used them. Try the owners clubs as they may have an exchange scheme.