Sorry, no go - the registration is for the car, not the engine and thus date of first registry for the chassis # determines whether you need a cat, no matter how old the engine you drop in there... With 'assembled' cars the age of the engine can make a difference though, for instance if the body/chassis is made up from two donor cars, and one of them corresponds with the engine and/or gearbox. So for instance if one half of the car is a 1985, the engine is from the same car, and th other half dates from 1992, the car will be registered as a 1985 (after a strict technical scrutiny, of course!).