So Alistair, is that 90% by number of parts, or by weight of parts? or by uniqueness?
Every vehicle is different and it really is down to the buyer to do his homework before he parts with his money.
"Based" is very broad. Is that like a new Spitfire built around an original turnbuckle?
To have a scale would then need adjuticators, like judges for conformity at dog shows. There would be no clarity.
Sorry Alistair I think your plan is flawed.