[NASSTY]

Plums,
The $70 test is not the full on emission test that you have done in the past. It's an at idle and 75% of Max power rpm only. A probe gets shoved up each tail pipe. The exhaust is already hot, no light up period. All the test does is confirms to the engineer that the ECU and related gear are all working as per factory specs. If the engine is not deemed to be compliant, ie from an ADR emission passed donor vehicle, then you would need the full on test as you described. It only confirms all this to the engineer. Having new cats helps if any of the components of the engine are past there best as the cat, when hot, will absorb a great deal. A pod filter does nothing for emissions as if there is any increased airflow, the ecu will compensate just as it is designed to do. The reason I think some engineers want you to use the airbox is due to noise, not emissions, as a lot of noise escapes from the pod filter.

In theory you can use any engine you want to but what stops you doing so is the prohibitive emission test Plums refers to. Using a donor engine from an ADR'ed vehicle saves you the trouble of full on testing as the engine has already passed the test at the factory, it's just been transferred into another vehicle.