After a detailed preflight inspection and a 15-minute run up, the 1,684-hour commercial pilot and a pilot-rated passenger, who had recently purchased the twin-engine airplane, departed on a 385-nautical mile cross-country flight to ferry the airplane to its new home base. About 25 minutes into the flight, while in cruise flight at 2,500 feet, the left engine lost power. The pilot elected not to secure the left engine as he felt that the engine was producing some power; however, he elected to return to the departure airport. En-route back to the airport, the right engine also experienced a partial loss of power. The pilot was unable to maintain altitude and selected a nearby hay field to execute a forced landing. The pilot was able to clear some powerlines short of his intended forced landing area, and made a normal landing on the hay field. During the landing roll on the hay field, the airplane impacted a 2-foot berm/terrace in the field, collapsing the nose landing gear assembly. Examination of the airplane revealed that fuel was present in the fuel tanks; however, water and corrosion were found in both left and right side gascolators.