The problem is that with the A-series usually needing a rebuild at around 100,000 miles, finding an undisturbed original installation these days is becoming harder. They do exist, but with even the newest of classic mini's being 13 years old now, it's often becoming increasingly difficult to find a classic mini still with it's original engine, let alone one that's engine is completely untouched. Also with so many classic minis now being owned by enthusiasts who want to keep everything under the bonnet clean and shiny, even genuine original installations will often be sparkly clean and fitted with a replacement chrome/alloy rocker covers etc.Conversely, a 25 year old mini that is had engine mods carried out a decade ago, might not be all that obvious when it's covered in 10 years of oil and road grime.
Engine number plates do genuinely corrode and break off too, just because you see a replacement engine number, or one that's been tampered with doesn't necessarily mean that anything dodgy has actually occurred. I've replaced an engine number plate because the original had genuinely broken off, and in my case the rivets could be prized out easily with just a screw driver. I do still have the original plate (somewhere), but whether I'd actually remember to pass it on to the new owner if I were to sell, is a different matter. With many classic mini's now having 10 or more previous owners, an engine transplant, or engine number change may have occurred many owners/years ago, possibly without the current owners knowledge.