I think the reshelling of a racecar such as this is somewhat different to reshelling a road car.
If, as the seller claims, it has been reshelled each time with new shells from Porsche (and not due to crash damage apparently, apart from the most recent reshell), then it's possible - and I stand to be corrected on this - that the new shells may have been provided by Porsche without chassis numbers, so the car could retain its original identity - or, at least, not have a conflicting identity when compared to the original shell.
However, to answer the original question, which was regarding what would be replaced during a reshell - well, there's no definitive answer to that, and it largely depends on why the car's being reshelled in the first place. In the case of this car, it looks like the first two reshells were simply to update the car to the latest specs for a particular series - different roll cage requirements, for example. So if that's the case, then it could literally be just the shell and associated parts that have been replaced and all of the major mechanical components may be from the original car. But in the case of a heavily crashed car, a lot more may need changing - suspension, steering, brakes, maybe even engine/transmission. Of course, in such cases, you've got to decide at what point it ceases to be a simple reshell and actually becomes an entirely new car!
People have been arguing about that one for years...
