On this parts fiche the Bolt,flange,6x32 is the most specified component.
A sample from the products for the Lower Crankcase include: Bolt Flange 6x40, Bolt,oil Filter, Bolt,flange,6x32, Pkg,oil Case.
The most reasonably priced component used on the Cb450s 1988 (j) Denmark Lower Crankcase is the Washer,plug,drain 12.