In my experience, Bureaucrats hate to be told they did something wrong, or re-working the mistakes of another Bureaucrat.
I would suggest a different approach. Tell them the engine has been changed. Offer up a certification from a local mechanic that the engine now in the car is in fact the 170 CID engine. How or when it got there is less important than the fact that it is there now.
Leave the model battle for later. The model does not cost you 1k a year.
Asking them nicely to correct changes to the vehicle may be a lot easier than getting them to admit a mistake. If there is any chance that they would have to refund taxes because of an "error" they will never admit to it.
B.