To make an appropriate tap, you need the cutting edges appropriately relieved so the edge can bite into the material. You need specialized tooling to do this—either a lathe with relieving attachment for an unhardened workpiece or a grinder that can relieve threads. Otherwise the tap will not be much better than taking a hardened 12-20 bolt and cutting some grooves in it. It is a non-trivial task.
If you wish to investigate further, you need to find some specialized books on cutting tool making. They are out there, and a good read.
(Yes, I am aware that 12-20 is basically a proprietary size that you cannot easily find—-that was kind of Stanley’s point. You have to decide whether keeping the hole original is worth the cost of special ordering a custom tap)
