The same piece on my VH snapped off exactly the same way. I contacted all the typical places in North America who repair Horseman Cameras and none of them had that part available. I had Japan Camera Hunter contact the manufacturer directly and he was told they no longer have any parts for those cameras.
At this point, since I've not been using my Horseman FA lately and have been using the VH, I temporarily transferred the locking lever from my 4x5 camera to the VH. Eventually, my (and your) only options seem to be buying and cannibalizing a "parts" camera or having some entity like S.K. Grimes fabricate a replacement lever.
Note that, while both levers are castings, the VH version is hollow where ours failed, while the FA's is solid and, therefore, more robust in that area. Should you seek a donor camera, the FA might provide a more durable solution.