Fabo: Northrop F-5 Freedom Fighters model E and F (single and twin-seater). I read they actually kept the black colour-scheme for actual combat training after the filming. The F-5 is also sibling model of the T-38 training jet that's un-armed, and related to the forward-swept winged X-29 experimental aircraft if I remember correctly. The plane is US made and as mentioned in the movie they were actually used alongside the Douglas A-4 Skyhawks to simulate agressor aicraft for the Top Gun pilots with their larger heavier Tomcats however if they had used F-5s in the movie then it would've been too clear it was the same aircraft as the fictional Mig-28.
And yes I've also heard that the Mig-factory only ever uses non-even numbers for the plane designations (Mig-15, 17, 19, 21, 23, 27, 29 etc). One site said that the moviemakers could've gotten an F-15 painted black with red stars on the tails to simulate a Mig-29 but they didn't. A "Mig-28" was totally unknown so it would be more mysterious.
Frank