Lyman
Maybe they could modify the old warbirds more, but I am sure that anyone racing these aircraft wants to reduce the risk to anyone from structural failure.
As far as new built racers go, look at what has happened to the purpose built racers like Miss Ashley and Tsunami. I think that the sport is running up against the limits of technology and engineering.
Unless they restrict the big boys like the F1 Air-racing, they'll probably keep much as they have been.