We upgraded the Cessna 421 with advanced avionics and systems. We installed modern flight displays, integrated autopilot, and updated GPS/NAV/COMM units, along with new transponders and multifunction displays. We also enhanced the aircraft with improved standby instruments, weather radar, USB ports, antennas, and connectivity solutions. Additionally, we performed wire harness installations, weight and balance checks, pitot-static testing, magnetic compensation, fuel tank calibration, and conducted thorough flight tests to ensure optimal performance and safety.