Kind of hard to diagnose what I would do by looking at a picture. But I think I would braze it and give it a try. First I would need to take the headset apart and do some inspecting to see how everything appears structurally. I don't know if that is necessarily the best option. Drilling a stop and a mig tack might work better. I would need to discuss it with a welder.
Of course, if the top cup is not loose in the head tube, riding and seeing if it gets any worse may work too..
