Russia is also ex-communist country selling new modern weapons worth billions of dollars. They also service and modernise old planes such as MIG-29 and SU-27. Belarus sold old Soviet weapons to middle eastern countries. Ukraine is a different story. They mismanaged their weapons neglecting their army. I’ve seen pictures of Ukrainian tanks left out in the open through which trees were growing. Many ships in their Black sea fleet was covered with rust. But those signing contract from Croatian side are partly to blame. The terms of contract should be such that Croatia may be able to return not operational planes. Croatia probably was lured by super low prices for those planes.
I am surprised MIG-21 (used in Vietnam war) are still purchased. Why not MIG-29 or SU-27? Both are modernised with modern electronics . In my opinion SU-27 is a work horse.