On August 6, 1999, at 1530 central daylight time, a Cessna 150M, N3755V, operated by Jolliff Aviation was substantially damaged when it experienced a loss of engine power on takeoff and nosed over in a soft field after a forced landing. The 14 CFR Part 91 flight had departed Sikeston Memorial Airport, Sikeston, Missouri, on a local instructional flight. The Certified Flight Instructor (CFI) and student pilot were not injured. Visual meteorological conditions prevailed and no flight plan was filed. The CFI reported that the engine was operating normally during run-up and takeoff. He reported that at about 200 feet above ground altitude, the engine started running rough. He applied carburetor heat and nosed the airplane over to maintain airspeed. The airplane continued to descend even though the pilot had applied full power. The CFI decided to land the airplane in a field at the end of the runway. He used full flaps and landed the airplane in the soft field. The airplane nosed over during landing rollout. An inspection of the engine revealed that one of the cylinders had an exhaust valve stuck in the open position. The owner and operator of the airplane reported that during the last 100 hour inspection, the cylinder with the stuck valve had a compression reading of 68 psi, just slightly lower than the other three cylinders.