On this parts fiche the O-ring,14.7x3.7 is the most specified component.
Some items from this specific schematic: Pipe,oil, Pipe Comp,oil Rel, Dowel,pin,8x14, Boss, Oil Filter.
The most reasonably priced component used on the Vt1100c2 Shadow 1999 (x) Finland / Kph Oil Filter/oil Pump is the Bolt,flange 6x30.