You can get a set of spark plug non foulers from the auto parts store that will screw into the bung. Go get HELP! brand spark plug non foulers part # 42009 or 42002. Drill out tip of one with 1/2" drill bit. Screw them together and then into O2 bung hole on pipe. This creates an extension to move the O2 sensor back.
The only difference in the part numbers is one of the sets is tapered, and the other is not. Either will work just make sure you drill out the top one to 1/2" so the sensor doesn't bottom out.