what was the reg? I can get some info from that re build dates. whole car though, not specifically the engine.
Pretty sure for the IVA it has to be engine age - cert of the car build date doesn't work .. I think, anyway. The engines I'm using are old enough that there's long since not been a V5 associated with the engine as the engines are older than me

The IVA manual now states:
Pre 1st August 1975 (all mine, then) are visual smoke test only
Before 1st August 1986 gets CO <= 4.5%, HC <= 1200ppm
1st August 1986 to 31 July 1992 gets CO <= 3.5%, HC <= 1200ppm
1st August 1992 onward gets a CAT test, CO <= 0.2%, HC <= 200ppm, Lambda between 0.97 & 1.03
If it fails to meet the BET limits (above) for 1st August 92 onward then it gets a CAT1 test;
1st Aug 92 to 31 July 95 with no match in the Emissions book gets a non CAT test (3.5% CO, 1200ppm HC)
If there is a match and the engine didn't have a CAT when built it gets a non CAT test
If there is a match and the engine DID have a CAT when built it gets a CAT test
There's a set of big long flowcharts in the IVA manual that make this a lot easier to follow!

AFAIK engine age must now either be proven with a letter from the manufacturer based on the engine number or;
Certification letter from a VOSA listed engine builder (of which there are only about six..)
The IVA manual just says this:
Conversions that are not based on M1 vehicles may comply with the applicable requirements of the original base vehicle. Documentary evidence of base or incomplete vehicle approvals will be deemed acceptable, Modification to the exhaust system length after the rear silencer will be permissible without further test
But I know people who've been knocked back without written proof based on the engine number either from manufacturer or (in the case of Chevy 350 blocks) a written cert from an engine builder (as there are a great many Mexican blocks bearing casting dates pre 75 but actually made up to the late 1990s..)