manifold heat control valve

I would get it working if possible just to have better warm-up performance if nothing else. If not, turn the shaft counterclockwise (standing in front of the car looking toward the rear) to open it up. Make sure the shaft is turning and not just the weight. If it's frozen, use some WD-40 and a small hammer to tap the shaft back and forth in the bushings to free it up. I wouldn't weld on the manifold. Those things are crack prone to start with. If you really have to remove the whole heat riser, I'd remove the manifold from the engine and grind the welds on the flap, drive out the shaft, and tap the bushing for a plug or remove the bushing and install a steel freeze plug.