just stick a cockpit in it :) 
there's a "fly all Migs" mod around though which does that for you. 
  
Oh, look - it's in exactly the same sticky ...  
"Flying New (previously non-flyable) Aircraft 
Previously non-flyable aircraft can be flown as follows. MajorLee has a flyable SU-7B. 
Say you want to fly the Mig-21Mf, which is in \Strike Fighters\Objects\Aircraft\MiG-21MF 
Open the *.INI file in that folder. It will look like this: 
  
[AircraftData] 
AircraftFullName=MiG-21MF Fishbed-J 
AircraftDataFile=MiG-21MF_data.ini 
LoadoutFile=MiG-21MF_loadout.ini 
  
Change it to look like this: 
[AircraftData] 
AircraftFullName=MiG-21MF Fishbed-J 
AircraftDataFile=MiG-21MF_data.ini 
CockpitDataFile=F-104G_cockpit.ini 
HangarScreen=MiG-21MF_hangar.bmp 
LoadoutImage=MiG-21MF_loadout.bmp 
LoadoutFile=MiG-21MF_loadout.ini 
  
The MiG-21MF will use the F-104G cockpit. 
Now use the hangar.bmp and loadout.bmp files from MajorLee’s flyable Su-7, and rename them MiG-21MF_hangar.bmp and MiG-21MF_loadout.bmp, in the MiG21MF subdirectory. 
That’s all. The MiG-21MF will now be selectable. "