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Your codes are setting because the valve timing is incorrect.
Also explains hard starting and low power.
Was the engine still running normally before you pulled it apart?
Did you follow any procedures or just guess?
You may have bent valves as well now.
Would be wise to take it to an MG specialist.
Your technician that has verified you have no bent valves should have no trouble verifying the valve timing.
If you haven't got a factory shop manual or at least an instruction that might be with the new belt, then you should have got some info before doing this job.
I have searched and it doesn't appear to be on the internet.
Sorry I can't help further.
I hope you get your car sorted.
I will opt out and open the question to the other techs if you wish.
I see nobody else has helped you as yet.
This video may help.
Rover and MG share the same engine across a few models.