I am far from any kind of IT expert, but:

From the GCR:

To maintain the stock basis of Improved Touring, updating and/or
backdating of components is only permitted within cars of the same
make, model, body type (e.g., sedan, station wagon, convertible, etc.),
and engine size as listed on a single Improved Touring Specification Line.
[/b]
While the quote is valid, you have to expand further. The 1.6 and 1.8 Miata's are on different lines in the ITCS so the swap is NOT legal.

AB