The pilot and his two passengers were in cruise flight on a VFR cross country when the airplane's engine suddenly began to run rough. The pilot, who was being flight followed by an FAA center, declared an emergency and requested vectors to a nearby airport. Within about 2 minutes the pilot reported that the engine completely quit. The pilot entered a power off forced landing and collided with hilly terrain. No suitable terrain was available for the forced landing and the lighting conditions were reported as 'dusk to dark.' The pilot and one of his passengers received fatal injuries. An off site examination and teardown of the engine revealed that the number 5 cylinder exhaust valve stem broke just above the valve head. The valve head fell into the cylinder and the action of the piston caused a hole to be broken through the piston center crown structure. The valve head was found wedged between the crankcase structure and the crankshaft, preventing crankshaft rotation.