What was found
AFTER TAKEOFF, THE RIGHT ENGINE STARTED RUNNING ROUGH. PILOT RECEIVED CLEARANCE TO LAND. AFTER LANDING, PILOT PERFORMED MAG CHECK TAXIING BACK TO GATE. LEFT MAG WAS DEAD. MECHANIC CHANGED MAGNETO, TIMED TO ENGINE, RAN UP GOOD. ON DISASSEMBLY OF MAG, FOUND DISTRIBUTOR GEAR MISSING 5 TEETH. GEAR WAS INSTALLED 6 YEARS AGO.