I have a J. Stevens Arms 12 gauge break-open single shot that the firing pin is broke on.
Do you have the original firing pin? I own a machine shop, it'll be simple to make a new one out of a scrap piece of tool steel (drill rod) as Clark says if you have the dimensions.
Last one I made for a Stevens was about 20 years ago. I inherited a 311 double barrel with only one barrel that worked when my father died. He used to talk about hunting with it when he was growing up back in the 1930's with the one working barrel. Turned out it needed a firing pin. I try to take it out shooting with my kids a few times a year in his memory.