This is a wonderful discussion, and one that should be taken very seriously.
One area we could look to for guidance is the Fine Art and conservation professions. It is highly likely that over decades or sometimes hundreds of years, paintings and sculptures and other artifacts have been repaired and/or restored. The art market has protocols established to acknowledge and document such work. Often the value of a work is directly attributable to the extent of later work (and the expertise) inherent in that work. Frequently when an artwork is sold the additional work is added to the "Provenance" (documentation).
Having said that; the art market is no place for the foolish or naive. Knaves abound, and try to pass off artwork that has had extensive repairs, as original work. When this fact is discovered, the dispute then begins. This sometimes results in the vendor refunding the purchase price, the auction house buying the painting (to preserve their reputation, as they should have checked the piece), or, the commencement of legal proceedings.
Herein lies an eventual reckoning for the car industry. With such large sums of money changing hands, for what has now, for many, become an investment strategy, the buyer will expect that he has paid for what he believes is either the original article, or that such modifications or repairs as necessary, to make the vehicle what it is, have been fully disclosed to him.
In disclosing the repairs the seller must accept the price may not reach that of an original car, however he rests easy. As many an art dealer has discovered, it doesn't take too long before the legal bills surpass the profit made. Profit gone and reputation damaged, the whole exercise then becomes pointless.
The art industry has one advantage over the classic car industry, and that is; centuries of academic and scientific research. The art world has established protocols in what constitutes original and altered, and has an enviable valuation process which determines the likely value of an artwork.
If the classic car industry desires such a system it could do no better than using the established, tried and tested mechanism's or the art world. I have heard that processes similar to those I have mentioned, are adopted at some very high end auctions, however that may be due to the auction house ensuring that honest vendors are given the nod, while fraudsters are sent packing.
Regardless of where each man stands on his interpretation of what constitutes an original car, he could show no better act of faith and honesty, if he fully discloses what he knows about his car when he offers it for sale. in doing so he allows the buyer to decide where he stands, he allows the buyer to form his own opinion, and then his interpretation of what constitutes an original car.
One way to attempt a degree of honesty (for current and future reference) would be to list the modifications and repairs on the Bill Of Sale, and make a small notation on the Title that a certain document should be read in conjunction to the Title.
And then there are the guys that don't care all that much, or can't be bothered...