On this parts fiche the O-ring,15x2.5 is the most specified component.
Parts such as the Gasket Kit B: Gasket,l Cover (mca), O-ring 12.5x2.2, Pkg,oil Cooler (nas), O-ring,3.7x1.5 are included in the Cb450s 1986 (g) European Direct Sales Gasket Kit B parts fiche.
The most expensive part in this assembly is the Gasket Kit B.