This is a crazy thread. The only two possible answers are: 1. A new barrel and a new gas block. -OR- 2. Get and adjustable gas block. That's it.

1. Welding a barrel is crazy and could cause weakening or warping of the surrounding material. Kinda red neck shade tree gun smithing IMHO.
2. Putting a set screw in is even crazier.
3. Using the gas port reducer might be a solution, but it might cost as much as a new barrel.
4. If you do get a new barrel, you'll probably have to get a new sight/gas block, as it is very difficult to line up drilling for a taper pin through an existing taper pin hole.

5. An adjustable gas block is probably the cheapest way to solve it legitimately.

This is why I won't buy used weapons...