THE PILOT STATED WHILE OPERATING IN CRUISE FLIGHT AT 9,000 FEET ON AN IFR FLIGHT PLAN, THE ENGINE '...GAVE A JOLT AND FAILED.' HE REQUESTED AND RECEIVED VECTORS TOWARDS THE NEAREST AIRPORT. HE REPORTED HE BROKE OUT OF THE CLOUDS AT 1,700 FEET MSL OVER HILLY WOODED TERRAIN. HE MADE A FORCED LANDING IN A SMALL, MUDDY CORN FIELD. THE AIRPLANE NOSED OVER DURING AN INTENTIONAL GROUND LOOP TO AVOID TREES AT THE END OF THE FIELD. POSTACCIDENT EXAMINATION REVEALED THE LOSS OF ENGINE POWER WAS DUE TO A FAILED EXHAUST VALVE IN THE #2 CYLINDER. METALLURGICAL EXAMINATION OF THE FRACTURED VALVE REVEALED FATIGUE CRACKING INITIATING FROM MULTIPLE ORIGINS ON OPPOSITE SIDES OF THE FRACTURE FACE. THE ENGINE HAD 1869 HOURS TOTAL TIME, WITH 671 HOURS SINCE MAJOR OVERHAUL. EXAMINATION REVEALED NO MATERIAL IMPERFECTIONS, AND THE MATERIAL HARDNESS EXCEEDED MANUFACTURER'S MINIMUM SPECIFICATIONS.