Sketchley's translation does talk about the source of the airframes, but it doesn't seem to confirm whether or not this particular pilot's (Magdalena's) is a surviving airframe, or if it's BASED off of a surviving airframe. It says that the SV-52's specs were not quite available. As Seto mentioned, there has to be some kind of market for aftermarket parts for that model. More than likely, it is possible to reconstruct a vintage aircraft this way. Maybe someone really liked the design and had the money to rebuild it.
All of this air racing is very much akin to our air races here on Earth, where the pilots use restored or kit-built vintage aircraft that are highly customized. It's kinda tough to find a P-51 that's in such good shape and is able to perform on such a high level these days.