Was broken when I bought Shamrock. Had plastic knob. PO was using plyers. Fortunately he didn't use it much. I used a metal screw and a made a plastic knob to repair. Has worked fine since, a dozen years or more. Think the problem was lack of use and got tough to operate, believe this switch/circuit breaker type needs to be exercised often to insure that it works when needed.
Please post if you find a replacement source. Admittedly my fix looks a bit cobbled together, it was. Stuff I had in my workshop.
FWIW This weekend was talking in an anchorage with a Beneatu owner, a former Catalina owner. He bemoaned the lack of parts and support from Beneatu.
Hope you find a quick, easy affordable fix.
Jim