Mercedes failed to repair my C63S

Mercedes Benz is failing to repair my car its been in their dealership for x4 months now without being fully repaired every time i quire am told that we are waiting for parts, how can you order 90% of parts to rebuild engine whereas you can just replace an engine they don't have AMG specialist engine was hand crafted and now it must be rebuild by people who is in experienced they can keep their c63s AMG i will buy other brands