If you really want to keep the gutted cat on there then try buying a new sensor. If the code is the heater circuit it might not have anything to do with the converter. But gutting the cat won't really increase your power in most cases. In fact in sometimes can rob you of power because the larger chamber inside a hollow cat will slow the exhaust gas exiting the engine down and can actually create backpressure. It's not a big deal on F/I engines since they are pumping out so much exhaust so fast, but on N/A engines gutting the cat is pretty much worthless.