heat and a few smacks with a pin punch should get the screw moving
replace more than a couple of screws and you will start to stretch beyond viability cost wise, far more fun to clean up the old ones with some small swiss files then cold blue them.
there are still a lot of spares out there in private collections but most collectors keep their dealers boxes to themselves....
"But we have our own dream and our own task. We are with Europe, but not of it. We are linked, but not comprised. We are interested and associated, but not absorbed."
Winston Churchill 1930