You would need a bottom tap, not a standard tap, also very expensive in that large of a size.
Possibly a local machine shop may have one and may attempt to fix it.
But even then there may not be enough material there to get any decent threads.
If you don't have the resources to fix it correctly(it looks bad), then options are few.
Replace the frame, a few hundred plus your time.
Or, at this point, I would use a small file and clean up the threads the best as I could.
Replace the nut if it is damaged, and if you can get the nut to get tight, or at least to the correct depth, by threading or just any downforce, then I would tack weld it on both sides.
There is very little up/down force applied to the steering stem aside from your grip.
Welding would hold it, the temporary heat near the bearing wouldn't ruin it, and the steering bearing typically has a long life, and is usually worn out by wet conditions before use anyway.
Without alot of options, welding will be the cheapest and easiest solid repair.