Guessing I may need to flash ECU?

Someone removed the main catalytic converter on my vehicle and replaced it with straight pipe. Two years later I have Check Engine light, 300 and 103 errors. At full throttle the engine suddenly goes to half-power. Stop, switch off, switch on again, and everything is okay ... till I use full throttle on a hill, at which point, the problem repeats. And is getting more frequent.

Have replaced fuel filter and spark plugs.

Am contemplating removing/replacing other two catalytic converters. Where I live we have no local emissions regs, so, this is possible. But also wondering if O2 sensor has gone bad. Or if I can just reflash ECU to tell it to ignore O2 sensors.