I do not believe this fact, the MiG-21 were not active, also an Albanian air force was grounded, their J-7 were not able to fly for years.
Two squadrons of MiG-21 were in an underground airport near Pristina, all survived the bombing, after the ceasefire was signed all flew to Batajnica near Belgrade.
https://en.wikipedia.org/wiki/Slatina_Air_Base
https://en.wikipedia.org/wiki/83rd_Fighter_Aviation_Regiment#1994-1999
https://en.wikipedia.org/wiki/Batajnica_Air_Base
https://en.wikipedia.org/wiki/NATO_bombing_of_Yugoslavia
In this forum, you'll find plenty of useful information:
http://forum.krstarica.com/showthread.php/109741-ISTINE-I-ZABLUDE-O-BOMBARDOVANJU-1999/page302?pp=15