If he used a new shell and sufficient parts from the donor RS2000 the car could be legitimate but, with all the Cosworth parts available why not just reshell the Cosworth?

Maybe he got a cheap Q Plate Cosworth and didn't want the Q Plate so settled for RS2000 ID but then, as a Cosworth shares little or no mechanical parts with an RS2000, he would have had to build the car using the donor RS2000 parts (assuming they actually fit?) to get the 8 points required, get it registered with a new DVLA VIN number and a new age-related number plate, then replace all the RS2000 parts with the Cosworth ones.