The accident airplane collided with a ditch during a forced landing. The landing occurred after the airplane sustained a partial loss of power in the right engine, followed by a total loss of power in the left engine. The unregistered airplane was being operated as a ferry flight. The pilot stated that the right engine began running rough about 15 minutes into the flight. About 15 minutes later the left engine stopped. He attempted a restart and was not successful. He reported he could not maintain altitude and selected a road for landing. When he observed vehicles on the road he selected, he ultimately landed in a wheat field. The airplane struck the ditch during the landing roll. After accident inspection of the airplane showed a defective right magneto on the right engine. A defective diaphragm was discovered in the left engine throttle body. A map review shows the pilot overflew two suitable airports after the right engine began running rough.