THE ENGINE LOST POWER AT 50 FEET AGL DURING INITIAL CLIMB AFTER TAKEOFF. THE ACFT DESCENDED, STRUCK A FENCE AS POWER WAS REGAINED, AND IMPACTED THE GROUND. AN FAA INSPECTOR WHO EXAMINED THE ACFT REPORTED THERE WAS NO EVIDENCE OF MALFUNCTION OR FAILURE OF THE ENGINE BEFORE THE ACCIDENT. THE OPERATOR REPORTED THE LEFT MAGNETO FAILED.