I can only provide feedback on choice number 1 above. Since it's a stainless header, if you're going to repair it by re-welding, it needs to be done by an experienced TIG welder using the appropriate filler rod, with Argon back-purging (purge the pipe of air by replacing it with Argon), in order to prevent stress-corrosion cracking. With thin tubes, the inside of the pipe becomes hot enough that the oxygen in the atmosphere pulls chromium out of the alloy. Chromium is the main corrosion-inhibiting alloying metal in Austenitic stainless steels. Without the chromium, the heat-affected zone (HAZ) will corrode quickly in an atmosphere of exhaust gases at high temperature. In addition, with Austenitic stainless steels, the HAZ is considerably smaller than that of plain steel because stainless transfers heat much slower than plain steel does. A smaller HAZ means higher stress-concentration. In addition, the thermal expansion of Austenitic stainless steels is appreciably higher than that of plain carbon steel, providing a double-whammy (that's a technical term) for failure in the HAZ. Welding your header with steel wire using a MIG gun and no back purging would be a complete waste of time, I'm afraid.