Exhaust pipe

Studs comes in two types, one type is screw on and the other is welded on. If it is screw on type, you need to remove it by means of a good vice or clamp, which at times is difficult. You might need the aid of heat or weld something on as a handle.
If it is the welded on type, you need to grind off the welded part to remove the stud. Drilling might be necessary in some cases. You can use a bolt and nut as substitute for re-installation, it is not necessary to weld on the bolt if you are able to hold the bolt for tightening.
This re-installation method is also applicable for the first scenario if you cannot find the correct stud.