There can be many reasons. Engine is one of them. Let us not says that all planes are going down due to engine.Its not about reviving Migs but saving pilots, Each year at average of two-three MIG-21s are going down, Mostly because of faulty engines ..
The best solution remains replacement of Mig-21 with Tejas, by massive ramping up of LCA Tejas volume.
However I am all for very careful maintenance of Mig-21 fleet. IAF should devote extra resources to Mig-21 maintenance to make sure that planes are flyable.