Well the simple answer would be that if its a built motor, its probably going to be driven harder than a stocker. I would venture to say that most built motors are either blown up for various reasons, sold because interests change, or rebuilt for a different goal before anyone can find out about their longetivity.

In some small way, I have to think that a built motor should last every bit as long as a stocker, but I've never had the occasion to find out...yet