The private pilot and a friend were enroute for a hunting trip. Thirty miles east of a refueling stop, the pilot heard a loud knock. Oil covered the windscreen and the engine seized. The pilot looked out the side window vent to select a forced landing area. A wheels-up landing was performed in a grain field. After exiting the aircraft, the pilot saw that the propeller was missing. Damage was reported as substantial, but a post-incident inspection revealed minor damage. The propeller was not found but two sheared propeller bolts, P/N A-2040, were found in the cowling. In a phone conversation, a Hartzell technical representative said the proper bolt for this installation is P/N A-1328-1. He said P/N A-2040 was superseded prior to 1970. Log books show this propeller was overhauled 83 flight hours prior to this incident. The repair shop reported that the bolts they removed were inspected and re-installed in the hub.