The O2 sensor voltage should fluctuate. If it stays steady in PSCAN, but fluctuates while measuring with a multimeter, I would check the connection wires from the O2 sensor to the ECU connector, perhaps the wire is broken, or too much resistance in the connector. You can also clean the connectors, including the ECU connector, with contact spray.
I think you cannot use a VVC ecu on a standard engine, even swapping two identical ECU's may not work due to not matching security codes with the instrument cluster of ingnition key. But I have no experience in ECU swapping on my MG, I did encounter this on my Audi.

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