Making a firing pin for one of those old shotguns or rifles is one of the simplest and easiest of gunsmithing jobs. But you do have to know what you are doing, no matter what material you use.
A word of advice. In many cases, the firing pins of those guns were removed or broken deliberately because a smart ancestor realized the gun was dangerous to fire and deactivated it. I have refused to replace firing pins on junker twist barrel shotguns because they would have been dangerous if fired, and been berated by "customers" who demanded I fix the old gun so they could go duck hunting using 3" Magnum shells.