Shortly after level-off to cruise, the engine began to lose power. The pilot attempted to restore the power, but the engine RPM continued to decrease. A turn was made back toward the airport. While scanning the instruments, zero oil pressure and a high oil temperature were noted. Shortly thereafter, the engine quit operating and the pilot was unable to get it restarted. He landed in an open field, and during the roll-out, the aircraft hit a ditch and nosed over. An investigation revealed that the engine oil pump (LW-12897) had failed. A teardown of the pump revealed that a tooth had failed and had become wedged in the gear housing. Also, the inner walls of the gear housing were scored. AD 81-18-04 applied to this engine and pump, but would not have become applicable until 2000 hours after overhaul or until the accessory section was removed. When applicable, it would have required the replacement of the iron sintered impeller with a hardened steel impeller. The engine had 1410 hrs, since overhaul.