I suspect the original will be simply a medium to high carbon steel (tool steel or silver steel) heat treated after machining the profile of the sear and cutting the thread.
If you wish to attempt a repair with hard facing build up the worn areas with a hard facing alloy, I would suggest a nickel based alloy suitable for impact use rather than a super hard cutting tool re-facing alloy of cobalt/tungsten carbide. Heat treatment after welding would be advisable as you are only working on a small mass of parent metal and a very small mass of deposited metal. Cooling too quickly will inevitably affect the hardness of the facing material probably making it brittle, cooling at the correct rate will give a tough rather than brittle deposit with a good carbon/nickel dilution through the heat affected zone. Deloro 40 is a good example of such alloy.
If using oxy acetylene then simply building up with a mild steel filler rod but using a carburising flame is a reasonable method, build up, harden then temper to 230-250 deg C.
Another approach would be to case harden after building up with mild steel, the case hardening will give the hard wear resistant surface but you may need to re-finish the round section after case hardening.
If making a replacement I would try silver steel as although it is simply a high carbon steel with no fancy alloying elements it can be hardened and tempered quite easily, it is cheap to buy and easy to machine in the annealed state. I would harden then temper it to a non-brittle state, approx 230-250 deg C.