During initial climb, the airplane's single engine experienced a partial loss of power. Unable to maintain altitude the pilot performed an emergency landing to a field. During the landing rollout, the airplane impacted a rock and the nose landing gear collapsed. The airplane came to rest in a nose down position and the pilot was able to exit unassisted. Following the accident, an examination of the airplane revealed that the firewall had sustained structural damage, and the throttle control rod-end to carburetor throttle arm hardware was missing. In addition, the airplane had not been inspected in accordance with an annual inspection during the previous year.