Um that's illegal. Mileage is for the vehicle, not the motor or any other part.
So swapping motor for bad or better doesn't constitute the turning back the odometer. At least not tell anyone about it.
Say if you just paid a whole lot of money for a pristine 1500 mile, garage kept, original tires, interior, paint 62 Corvette.
But the original owner lied, and took a wrecked burnt destroyed car and made it into the vehicle you just bought for 300k.
How would you feel?
It would devastate the resale of vehicles.
My 1960 Ford f350 barn find is used, but the odometer is intact. 71565 miles I know that its has twice as much. I think it the late 60s they started adding the 100,000 odometer.