That isn't a crack, it is a complete failure. You can see where an attempt was made to weld it before. You should check your engine mounts, and ensure that the exhaust brace at the bell housing is secure.
It would be possible to slide a short length of tube on to the pipe and then line them up and do two continous welds around the new section. Cut out the section with the bung. Other than doing that, welding what you have is a waste of time.