Another way to do it is to find a nut that will sit on the head of the rounded bolt. Weld through the hole in the nut onto the head of the bolt (sometimes a challenge for me to get good penetration on the bolt head before getting the nut filled, but I'm not an expert with the welder) to put a new head on the bolt. I suspect that the heat cycle from the welding may also loosen rust in the threads. Before doing this think about whether there are parts in the area that might suffer from the heat of the weld.
