The Romanian MiG-21s [0], Polish MiG-21s (when in service, before retirement) and MiG-29s along with the Bulgarian MiG-29s were upgraded to NATO standards. The primary upgrades were the radios and IFF transponder. I'm not sure if they all included

It is widely rumored that certain spare parts for NATO MiG-29s are actually made in US, from sources initially supporting the Adversary training missions of the 57th Wing Nellis AFB and US Navy Adversary training missions, including TOPGUN department. This is primarily for the MiG-29 and Su-27 fighters.

[0] https://en.wikipedia.org/wiki/List_of_Mikoyan-Gurevich_MiG-2...

[1] https://en.wikipedia.org/wiki/57th_Wing

[2] https://en.wikipedia.org/wiki/Naval_Aviation_Warfighting_Dev...

Well, when talking about NATO countries, MIGs and older aircrafts - Romania and Croatia still operate MIG-21s [1] :)

And MIG-29s are present also in the Bulgarian [2] and Slovak [3] airforce too.

[1] https://en.wikipedia.org/wiki/Mikoyan-Gurevich_MiG-21#Curren...

[2] https://en.wikipedia.org/wiki/Bulgarian_Air_Force#Current_in...

[3] https://en.wikipedia.org/wiki/Slovak_Air_Force#Current_inven...