On this parts fiche the Seal,valve Stem is the most specified component.
This schematic depicts all components used in the Cbr600f Hurricane 1989 (k) Germany / Kph Gasket Kit A partslist.
The most reasonably priced component used on the Cbr600f Hurricane 1989 (k) Germany / Kph Gasket Kit A is the O-ring,15x2.5.