The valve seats are integral with the cylinder head. The seats are machined into the cylinder head casting itself. On the aluminum heads, the valve seats are separate inserts, which may be replaced by a competent machine shop if required. The reason for using inserts on this engine is basically because the aluminum could not withstand the constant pounding of the valves opening and closing as well as the hardened steel insert which is used.
Machining of the valve seats, or replacement of the seats in the case of the diesel, should be referred to a professional machine shop.