According to the pilot, the left engine began running rough at about 300 ft AGL after takeoff and the roughness was accompanied by a slight left yaw. The pilot said the left engine was producing some power so he elected not to feather the propeller. The aircraft would not maintain altitude; therefore, the pilot elected to make a gear-up landing in a wheat field. An investigation revealed the plane had been involved in a previous gear-up landing and had accumulated 420 hours since then. An examination of the left engine revealed that the propeller would rotate freely without rotation of the crankshaft. Examination revealed that the spline bushing, PN 75462, turned freely in the front of the crankshaft, and the three dowels, PN 75359, were not in place. The bushing, which is pressed into the crankshaft during assembly, was loose enough to permit removal by hand. The dowels were found in the engine oil sump and were bent and deformed.