The sport pilot was departing the 1,200 foot grass strip. The pilot reported that the takeoff was normal until passing 250 feet above ground level when the airplane's engine began losing power without any associated roughness or engine vibration. The pilot did not observe the gauges and focused on finding a suitable landing area. Since the airplane's flight path was towards slightly rising terrain, the pilot elected to turn the airplane towards a suitable landing area. During the turn, the airplane began losing altitude and the pilot performed a forced landing to an uncut hay field. Despite trying to keep the wings level, the left wing struck the hay first resulting in the airplane pivoting around the left wing tip. The airplane rotated 180 degrees before coming to rest in the upright position. A fire started near the lower portion of the engine and the passenger assisted with the pilot's egress prior to the airplane becoming engulfed in flames. Due to the remote location, the pilot reported that the fire response took 30 minutes. The pilot reported that the engine had operated normally prior to the accident flight without any indications of an impending failure. Thermal damage prevented a thorough investigation of the engine and the reason for the partial loss of engine power could not be determined.