The other thing is what are the legal requirements for changing engines in Canada. In some states(US) it is illegal to swap an older engine into a newer car for emissions reasons.
When in actuality there were years of a given engine that there really was no difference in how much an engine (at least the long block portion) of one year vs same model engine of another of another year would mean a damn in emissions output.
Plus if the older engine has less miles, is in better condition, possibly overhauled, or a million other (I'll call them "excuses") the older engine could possibly run cleaner than the one being replaced.
Alot of the emissions crap was just "bolt in" anyway over the years