No, I'm not going into some hi-cap business; just wondering how one can use a press, slip roll, shear, and brake to restore antique firearms into service.

Most mags are more complicated than can be formed with your tools.  Usually you need a metal stamping press with special dies that cut a magazine out, sort of like using a cookie cutter.

Once you get the mag stamped out you'll require forming dies and for that you usually need a milling machine to make the dies.
Most mags would require multiple forming dies to form the lips and the body.  Much of that requires several dies for each operation, since most of it can't be done in one pass.
Again, for the forming operations you'll need a stamping press.

To finish most mags you'll need welding technology to join the seam.  
Some are spot welded, some are fusion welded down the seam and the welders to do that like Tig/Heli Arc aren't cheap.

With the exception of the M1 en block clip, I suspect that clips would be easy to produce but...why would you want to?

The price of magazines or loading clips can equal the cost of the firearm, and if one can replace a barrel and have it as accurate as it ever could be, why shouldn't we make the feeding devices?

Usually what antique gun owners are told is that "it's not your ammo; must be the mag..."

Everybody knows how to swap followers and springs, so why don't we just cut and squeak the lips, if that is the problem?

Many guns are actually designed around an existing proven magazine design.

I've seen some 3d printed low capacity AR magazines but they didn't hold up well. But that was a while ago, things may have gotten better.

They did.
Brownell's sell magazine lip forming dies to repair 1911 magazines.

Many modern magazines are plastic or aluminum.
Neither take well to trying to bend them around.  Usually, once damaged, that's it, the mag is scrap.  
Pistol mags of steel could be fixed, but how many different die sets are you willing to manufacture to cover all the different magazines being made?

Then too, most people would not buy such a die set, so you won't be able to stay in business very long.