From the info I have it would be tested with the following standard

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.

Hopefully a current tester can say if this is still in the current tester manual.