There are a couple of ways you could go about this,
You could use an Ego clamping feedneck which would involve turning the cocker body on a mill or lathe removing the thread on the feed neck cut a little off the top with a slitting saw if you don’t have access to the machinery use a file.
Or
FBM do the press fit clamping feedneck
these here which would involve diilling a hole through the feedneck stick the body in to a vice make sure you use soft jaws stick a bar or larg screw driver through the hole and slowly lift and twist. It will take time but will pop out after a while. Then carefully press fit the new feedneck in place using the vice. Or you could send it to FBM in the states and have them do it.
Ive tried both types on cockers and the press fit on an X-Mag vert module and must say its worked out pretty well.
Hope this helps