During an IFR flight at about 11,000 feet MSL, the airplane's engine began to run rough and the engine oil pressure dropped to zero. The pilot also said he smelled something that was burning. The pilot entered a descent and requested vectors to the nearest airport. While on the vector, the engine began to run rougher and the burning smell became stronger. The pilot shut down the engine and entered a power off forced landing. He broke out of the clouds and determined that he could not glide to the airport. He performed a landing to the only available area. During touchdown, the airplane collided with posts. An FAA safety inspector examined the engine. He determined that 1 of the 4 bolts used to bolt the engine oil pump cover to the crankcase was missing. A damaged bolt was found in the engine's camshaft gear. The tab washers used to lock the oil pump cover bolts had their single tab bent downward. The tab washers' twin tabs on the 3 remaining bolts were pointed straight out. They appeared to have never been bent into place after the bolts were tightened. Improper installation of the washers resulted in one bolt falling off and being ingested through various drive gears. The engine was overhauled by the manufacturer 515 hours prior to the accident.