How so? You already have the flight body needed, just design a control system and replace the pilot controls with it.
Military aircraft of the 50s and 70s are not obsolete. It is continually improving. China can integrate AESA radar into the Mig-21 if needed. But the better way is to convert the Mig-21 into a UAV.
Replying to both.
Point is that if you want to convert an old fighter type into a UAV the roles of that UAV will be fairly limited.
In peacetime it will only be appropriate either as target drones (expendable) or as part of technology demos in pursuit of more advanced and proper, capable UAV or MUM-T.
In wartime it will only be useful as expendable decoys or diversions.
However conversion of an old manned aircraft would not be useful as proper future proof, modular, sensor or shooter equipped UAVs that were designed and built from the ground up for such a role.
What you're describing is basically just a glorified target drone or decoy aircraft at best.
This is off topic and I'm not going to go further into it in this thread -- but there is a reason why nations with proper, capable aerospace industries who are pursuing future UAVs aren't just converting their old manned fighters into UAVs.
Because such conversions will be limited in capability and scope compared to what true, future, clean sheet UAV designs will be capable of and how they are meant to be operated or their cost.
If your goal is just to have target drones for use in peacetime, or in wartime sending some unmanned old fighters as decoys or diversions, then sure, fine. But a proper capable UAV it will not be.