i don't think the MiG-31 was ever used outside Russia/CIS. The '31 was the technological successor of the Foxbat, and i doubt that Russia/CIS would have exported such an advanced interceptor, even to its allies. After all, the Foxbat performed along the same lines, was less advanced, and was a 20 year old airframe when the Foxhound came on the scene. For variety's sake, you could create a bogus squadron of, say, and Indian or Egyptian '21s and '31s.
P.S. i found an interesting site that may be of some use:
http://www.aviation.ru/MiG/
If you're feeling adventurous, why not try and convert your MiG-21 into a twin engined E-152A! dunno if anyone's done that before...