Well, the welds would need to be redone, then the relief cuts machined out on a mill, then refinish the slide. Even then, since you put so much heat into it welding, the heat treat is possibly affected.
If it were mine, I would call HK and see what they offer to do and how much they want to do it. My bet is they would want to replace the slide, but they might want to scrap the gun. That's why you call first . . .