WHILE AT NORMAL CRUISE THE ENGINE BEGAN TO RUN ROUGH AND LOSE POWER. THE PLT DECLARED AN EMERGENCY AND EXECUTED A FORCED LANDING IN THE SPARSELY POPULATED DESERT. DURING THE LANDING FLARE THE PILOT MISJUDGED THE ALTITUDE AND LANDED HARD. THE TWO REAR PASSENGERS, THINKING THERE WAS A FIRE ONBOARD, RELEASED THEIR SEAT BELTS BEFORE THE LANDING AND WERE EJECTED THROUGH THE DOOR ON TOUCHDOWN. EXAMINATION OF THE ENGINE DISCLOSED THE SODIUM FILLED EXHAUST VALVE FAILED IN FATIGUE.