Seat rings for gate valves if not too badly damaged, may be repaired in the body by lapping.
The seats can be lapped in the body, using a flat lapping plate 1/4" thick with the OD 1/16" larger than that of the seat and having a 1/4 " high boss on one side 1/32 " small in diameter than ID of seat.
The plate must have a square hole in he centre for attachment to a square end tool.
Use a square tool of suitable size and length with one end to fit a brace and the other end attached to the plate