Why not unthread the operator part of an all-bress Champion antisyphon valve? Then you can thread in a new operator, like a Lawn Genie or Irritrol.
If the valve still is functioning, as far as turning on and off, then the operator is still good. The vacuum breaker part of any antisyphon valve will need service sooner or later, and you can find a kit of all the replacement parts you need to swap out, once you unthread the vacuum breaker cover.