Dan:
Most vintage race organizations, HSR, SVRA, HMSA, want vintage cars with original racing history. Lacking that they look for authentic, period-correct cars. In the case of GT40's there are so few of them racing that when they have a very close replica like Fran Kress's car or Bill Peter's red MKIV, they'll allow it to compete.
Modern replicas, whether they be GT40's, cobras, or jags or whatever, are usually not even close to having the original look or performance of the originals.
Most of the vintage race organizations also have engine, tire, brake, and suspension specifications, which state the cars must not contain modern technology, only the technology which was available at the time the car originally raced. That would exclude many of today's replicas which are fitted with radials, oversize rims/tires, modern brakes, stroker engines, power steering, power brakes, non-orginal types of transmissions, clutches, etc.
This however doesn't mean that the cars don't have a place. There are many enthusiast clubs and organizations which welcome the replicas on their track events , so long as they meet safety requirements. I'm even aware of many Porsche Club events which will welcome almost any type of car so long as the driver is properly trained.
I'd also have to agree with Johan. There are great drivers with big wallets, and great drivers with small wallets. And bad drivers come in all shapes and sizes. Bottom line...if you cant afford (financially or emotionally) to bang up your baby, whether it's real or a replica, you'd better think twice about taking it out on the track.
Mixing it up with some great historic cars, Monterey 2005.