All good ideas, but as Peter DeLorenzo says on Autoextremist" "ain'tgonnahappen.com"
I am sure that our friends in the UK will tell us how much of a sham the various historic racing certificates are. A noble idea, but it just does not work. The best system, such as it is, is what we have. The word of mouth and evidence of the various experts of various marques and series. If someone is pulling a fast one, there will be someone who will blow the whistle. In the current example of Scott Hughes, many people sounded the alarm.
As far as putting the burden on the promoters to only allow 'legitimate' cars, no way. First of all, the vast majority of promoters do not want to be put in that position. Further, face it. The promoter is mainly interested in the entry fee, not the pedigree of the car.
In Australia we have a "Certificate Of Description" (as well as historic logbook) which can be a lengthy and tortuous process but basically it tries to enforce correct technical representation of a car at a particular point in time. Of course this leads to some rather arcane arguments but the intent from everyone is to assist get cars onto the track for enjoyable amatuer motor sport BUT only in as much as they have correct specifications and have genuine proven history. For some this is a step too far but a simple example would be that the car must run the engine type and size as in the day. That would make most current Can-Am cars in America ineligable and the concept of "run what you brung" would be eliminated. Do those cars really need 9 litre engines today? F5000 means a 5 litre engine but apparently there is no appetite at all to enforce that type of very basic regulatory requirement in historic or Vintage racing there. Correct gearboxes, materials, instruments, wheel sizes, brake size and type, shocks etc soon follow and some countries have no chance of introducing this type of regulation retrospectively as the genie has long been out of the bottle. The show Vintage racing creates is very impressive and that success and enjoyment is a strong case to leave things alone and I understand that attitude completely.
The discussion of what represents a "legitimate" car needs to be had from 2 fronts.
1)Correct technical specification to best represent a car as near as reasonably possible for the period and
2)correct historical presentation of a car's competition record and driver/ownership in period and since.
These cars should outlive all of us and as custodians we have a duty (IMHO) to tidy up our act - one car at a time if need be.