are you aware part of upgrade package for F-20 was AN/APG-67 the Radar used in F-16A/B and that Radar can easily installed in F-5 , by the way the Radar in Kosar is far from the one in F5
by the way why you think the avionic must be old , Iran announced 4th gen Avionic . is it hard to believe by advancement in computer science since 1980 , it has become possible to design modern avionics that actually can be installed in F-5 Frames ? the problem with F-5 was limited space in its nose , otherwise it can easily house the engine of F-16 or JF-17 . and that limited space in nose were mainly a problem for 1960 and 1980 that electronics were a lot bigger than today .