[2001 ZTS]

EDIT: not necessarily illegal but "difficult":

What happens if I change(d) the motor in my Light-Duty vehicle?

If the vehicle is included in the program (appropriate age, registered in the program area), it will be considered a hot rod if the motor differs from the type that was originally offered by the manufacturer for that particular model year. All hot rods registered as the model year 1999 or earlier will continue to be tested according to 1980 emission standards. Vehicles registered as model year 2000 and later will be tested according to emission standards for their model year. Vehicles built to non-Canadian standards but imported into Canada will be tested similarly. Other specialty vehicles will be tested according to 1980 emission standards for an indefinite period, except for vehicles that have "kit" marked on the vehicle licence. Kit cars are exempt from the Drive Clean program.

It exposes a fault with DriveClean but if you think a V8 Mustang engine can pass the standard of a ZETEC then you may try. The fault is that anyone could have bought a "stinky" V8 but test at a lower standard than someone who bought a Focus. DriveClean measures emmision instant volumes not tonnes emmsion-mile/year.