Do you know a good blacksmith? He should be able to bend something for you.
In lieu of that, I am thinking Dixie might have something like that on hand. They are my go to place for unusual parts and pieces.
Another thought would be to get Track's catalog and see if they have a lock that approximates the dimensions of your rifle's. Could pursuade the new lock into fitting.
Another thought is that IIRC, that mountain rifle shares a bit of heritage with the Pedersoli rifles. You might want to see if those lock parts would be interchangeable. That is a wild, outside guess on my part.
Other than that, I guess you could loop a big rubber band around the hammer and hope for the best. :rotf
John