Brian Dudley in NY will know about specific Fox parts. Also Jay Schachter at VFI Guns -Grand Rapids, MI.

The recommendation to take the Fox carefully apart, with a smith -is excellent advice.
One can learn from a good gunsmith. Sometimes a good internal cleaning will fix the problem as crud or even a bit of rust
can cause problems in the action.

This is particularly valuable, as I own three AH Fox shotguns -all built in the teens or early 1920s.