How much of a car can be replaced?

In regards to selling VINs, which is questionable in its legality, how much of a car can be replaced before it is no longer the original car? We all know about just cutting out and welding in serial numbers, and most of us would consider that an attempt to defraud, but suppose the car was in a serious accident -- buckled sub-frame, crushed roof, front and rear damage, etc. -- could the car be repaired? Suppose most of a donor car was required? Where is the line drawn?

You have a K-code. Still got the original rear, original transmission, engine gone, but have an authentic replacement. Car is all there, but the shock towers are cut, cowling needs replacement, there is roof rust around the drain lip, trunk bottom is rusted through, one quarter panel has been cut off and repaired poorly from a parts car, the floor pans are all rusted through, water has rusted through the trunk seal channel. Other stuff like seats, fenders, hood, trunk lid, possibly doors will need replacing. Plus, all the other stuff that normally gets replaced like bumpers, trim, interior, etc.

Next to you is a much better parts car not needing nearly as much metal replacement. Its the same model, but an A-code. You own both cars. You could A) cut up the parts car and patch the K-code as best can be done, or move the K-code stuff into the parts car. In scenario A, you would still have as much of the K-code metal as possible, but many weld seams. In scenario B you would have a cleaner and more solid car.

This happens with many race cars due to wrecks. Just a little of the original remains, with most replaced. So, doors, fenders, quarters, lids, floors can be replaced, but not the unibody?

Disclosure is a good idea, but once the car trades hands, the next person is not obligated to reveal what he knows.