Modernization for certain aircrafts

Hey guys,

I have been doing some research on modernization of certain aircrafts for my marketing class and I am having some sort of difficulty in finding the target market for these air crafts, i.e. I can’t find the countries/operators of such planes, who would be needing the following aircrafts to be upgraded/modernized.

Su 24
Su 25
MiG 23
Mig 29

If anyone of you has any article(s), or related information about the target audience for this market, it’ll be highly appreciated and credited!

Thanks!
Lynx