as per the current regulations on engine emissions..
the engine you put in the car must meet the emissions standards for the year of production of the car.
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Just like to clear this up slightly, I believe draven is saying year of production of the donor car. So if you put a 1994 20v in your 1983 Sprinter you will have to meet 1994 emissions standards.
Just another little step to add to your project. Unless of course your car is from pre 1971!