Here are the current rules for emissions testing:
Before 1 August 1975: A visual test for excessive smoke.
1 August 1975 to 1 August 1986: Limits are 4.5% CO and 1200ppm hydrocarbons.
1 August 1986 to 31 July 1992: Limits are 3.5% CO and 1200ppm hydrocarbons.
After 1st Aug 1992 you need a catalyst and my eyes glazed over at that point.