mate they HATE exhaust pipes.... Even if you purge them and fill with Argon unless you have cleaned the soot off the inside the chances of the weld being chit are high. Can you easily get into the pipe to clean the spot near the crack?

You could look at welding another piece of stainless over the crack if its in a spot that cannot be seen. Also if you cannot find a welder per-sec, look for a Stainless Steel or marine fab shop as they often have welders on site as well. (I would offer to do it, but I'm not that good at stainless yet)

Call Matt at Krank Engineering 0437672700, He's a Bike Guy