Parts such as the E-8 Oil Pump: Pump Assy.,oil, Bolt,hex 6x28, Pin,dowel,8x18, Rotor,oil Pump are included in the Xr250r 1982 (c) E-8 Oil Pump parts fiche.
On this parts fiche the O-ring 7x1.7 is the most specified component.
The most reasonably priced component used on the Xr250r 1982 (c) E-8 Oil Pump is the Bolt, Hex., 6x35.