WHILE FLYING AT 8000 FEET MSL A PARTIAL POWER LOSS DEVELOPED. WHEN THE PILOT OBSERVED OIL ON THE LEFT WING STRUT HE CHOSE TO EXECUTE A PRECAUTIONARY LANDING IN A FIELD TO PREVENT ENGINE SEIZURE. DURING THE LANDING ROLL, IN SOFT TERRAIN, THE AIRCRAFT NOSED OVER. POST CRASH EXAMINATION REVEALED THE NUMBER TWO CYLINDER EXHAUST ROCKER ARM BROKEN IN HALF. METALLURGICAL EXAMINATION REVEALED THAT THE ROCKER ARM SEPARATED IN FATIGUE. THE ROCKER ARM(S), WHICH WERE NOT UNDER AN APPROVED STC, HAD BEEN INSTALLED APPROXIMATELY 11 HOURS PREVIOUS TO THE ACCIDENT.