I know there are probably some existing models of MiG-21 for FG, but not under GNU/GPL license, and thus not in the official game! I'm completly new in this type of modeling/programing but I'll give my best. Any sugestions/help are welcome:)
I could only find this one:
http://games.softpedia.com/get/Mods-Addons/FlightGear-Addon-Mikoyan-Gurevich-MiG-21.shtml
The basic model is finished, I'm working on textures. We'll see how is going to fly...