I was able to get castings of an original "Ketland & Co." marked lockplate, cock, frizzen, and frizzen spring from Jack Brooks recently to replace a missing lock for a fullstock fowler similar to yours recently. The top jaw and jaw screw were not included. I think Jack would sell you the cock casting by itself. You might have to modify the position of the square tumbler hole to fit your lock.
This is Jack Brooks' website:
http://www.jsbrookslongrifles.com/riflessundriesforsale.htmThe lock I'm referring to is the sixth lock down on this site.
This is the lock I made using his parts, and large Siler internals:
Gregg