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. "