find engine number then use the engine date table in the burton power catalogue/website to date your engine.

What is it on your logbook???

If its a car pre aug 1992 then no cat needed even if you fit a brand new modern engine.

If its a post 1992 car then there is a rule in the MoT test book that allows you to have "engine age" related emissions test, but a picky MoT man will want to see the correct engine on the log book to do it as well as proof of engine age. Its how you can get away with dirty cossie engines in new focus's etc, although to be fair the modern mapping skills of such likes as MSD etc means you can get a high power cossie through CAT mot tests no problem with the right map.