The rules appear to be clear but, do not specify if you can change engine makes all together. An idea I have it to take my entire drivetrain and stuff it into another chassis. In my case it would be a Mitsubishi engine into a Chevrolet chassis. Would this be legal for Blown gas coupe.
They record seems very soft. This might be bacuase you cannot swap makes limiting the creativity and capability of a car but, I could be wrong.