MLU = The jets have already gone through half their life, hence mid-life. Like F-16V, M2000UPG, Mig-29UPG etc.
Modernisation = New build aircraft with new avionics and perhaps even airframe and engine modifications. Like Su-35, F-16 B70, Super Hornet etc.
The cheapest and Quickest would be new MiG 29 airframes
Upgrade them , Put the Latest BVR and they can all replace MiG 21s