You don't have took to the America. Dakar does the same already. Anyone can build car there with variety of engines. And there are a lot of entries from various makers, privateers, or tuners.

For example, Martin Prokop is building his Ford Raptor in his team. Well if a privateers can build Dakar car, I'm sure that huge teams like Prodrive, GCK or Oreca will be able to build Rally1 car.