I split the head on a similar screw that was on an 1850s vintage Manton 10 gauge. Because of the non-standard threads that these old guns use I had my local smith cut a new roughed-in screw with the proper threads for me for about $40. I fitted the screw, indexed the slot and finished it myself. I used a power drill and a bench grinder to fit and finish the screw head and a hack saw and a small shop vice with pad jaws to carefully cut the indexed slot and cut the screw to the right length. The result was a beautifully fitted, perfectly indexed, and fitted screw. My work at home took maybe an hour. FWIW Im not a mechanic, machinist or engineer, Im a geologist who happens to enjoy working with his hands. If you dont have such an aptitude (and not everyone does), I suggest you bite the bullet and pay the price to someone who does.
