ACCORDING TO THE SURVIVING PASSENGER, A PARTIAL LOSS OF PWR OCCURRED DRG FLT & THE PLT TRIED TO LAND IN A FIELD, BUT THEFIELD WAS TOO SHORT & THE ACFT HIT TREES & CRASHED. DRG AN EXAM OF THE ENG, A METAL FRAGMENT WAS FND IN THE #3 CYLINDER.EVIDENCE OF FOREIGN OBJECT DMG (PEENING) WAS FND IN THE #2 & #3 COMBUSTION CHAMBERS & ON THEIR RESPECTIVE PISTON FACES. THE RESPECTIVE SPARK PLUGS WERE DMGD TO THE POINT THAT THEIR GROUND ELECTRODES HAD BECOME DISPLACED & SHORTED TO THE CENTER ELECTRODES. A METALLURGICAL EXAM OF THE FRAGMENT REVEALED IT WAS FM A BROKEN EXHAUST VALVE; HOWEVER, EXCEPT FOR PEENING, THE VALVES WERE INTACT. FURTHER INVESTIGATION REVEALED THE ENG HAD AN EXHAUST VALVE FAILURE IN 1980 & HAD FLOWN98 HRS SINCE THEN. PIECES OF THAT VALVE WERE NOT FND & WERE ASSUMED TO HAVE BEEN 'SPIT OUT' THRU THE EXHAUST SYS. DRG ANANNUAL INSPN ON 1/12/89, AN EXHAUST VALVE FRAGMENT WAS FND IN THE ENG MUFFLER. THE OWNER WAS NOTIFIED; THE EXHAUST PIPESWERE REMOVED & INSPECTED; NO OTHER FRAGMENTS WERE FND. THE ACFT WAS NOT EQUIPPED WITH SHOULDER HARNESSES.