INADEQUATE MAINTENANCE BY OTHER MAINTENANCE PERSONNEL WHICH ALLOWED A FOREIGN OBJECT TO REMAIN IN THE ENGINE AND DAMAGE/SHORT THE SPARK PLUGS IN THE #2 AND #3 CYLINDERS. TREES IN THE EMERGENCY LANDING AREA CONTRIBUTED TO THE ACCIDENT. IF SHOULDER HARNESSES HAD BEEN INSTALLED AND USED, INJURY(S) MAY HAVE BEEN LESS SEVERE.