THE PILOT REPORTED THAT WHILE IN CRUISE FLIGHT, A SEVERE VIBRATION BEGAN IN THE ENGINE WITH A CORRESPONDING REDUCTION IN ENGINE RPM TO ABOUT 1500. THE PILOT NOTED THAT ALL ATTEMPTS AT REMEDIAL ACTION MET WITH NO SUCCESS AND THE ENGINE WAS PRODUCING INSUFFICIENT POWER TO MAINTAIN FLIGHT. THE PILOT ATTEMPTED TO LAND ON AN INTERSTATE HIGHWAY BUT THERE WAS TOO MUCH VEHICLE TRAFFIC TO MAKE A SAFE LANDING. THE PILOT THEN LANDED IN A DIRT FIELD ADJACENT TO THE HIGHWAY, CONTACTED A LARGE BERM DURING THE LANDING ROLL OUT AND THE AIRCRAFT NOSED OVER ON ITS BACK. EXAMINATION OF THE ENGINE REVEALED HEAVY DEPOSITS ON THE INTAKE AND EXHAUST VALVES. THE EXHAUST VALVE FOR THE NUMBER 3 CYLINDER WAS IN THE CLOSED POSITION AND COULD NOT BE MOVED. DETAILED EXAMINATION OF THE NUMBER 3 CYLINDER REVEALED THAT THE ROCKER ARM PIVOT STUD FOR THE EXHAUST VALVE WAS BROKEN WITH SHEAR SIGNATURES EVIDENT.