Which MX5 donor?

[skov]

If you pick an engine that was manufactured in 1995 or earlier you shouldn't need a CAT to pass IVA or MOT emissions (regardless of whether the engine was fitted with a CAT in a different vehicle).

What I'm not quite sure about is what happens if you change to a newer engine post-IVA.
This is what it says about engine swaps in the MOT Testers Manual:

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Vehicles Fitted with a differnt engine

Test according to which is older, engine or vehicle.
e.g. A 1995 car fitted with a 1991 engine (of whatever make), test to 1991 standards for emission purposes.

Note: The onus is on the vehicle presenter
to prove engine age.
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So if you pass IVA with say a 1992 engine and no CAT, then swap to say a 2002 engine you may well need to fit a CAT to pass MOT emissions!