On June 28, 2007, at 1845 central daylight time, a Cessna 172, N6267E, operated by Air D Inc. as a rental/instructional airplane, sustained substantial damage on impact with terrain during a forced landing near Oak Grove, Minnesota. Visual meteorological conditions prevailed at the time of the accident. The student pilot reported he attempted the precautionary landing after the airplane engine began to run rough. The 14 CFR Part 91 solo instructional flight was not operating on a flight plan. The student pilot was uninjured. The flight last departed from St. Cloud Regional Airport, St. Cloud, Minnesota, and was en route to Backus Municipal Airport, Backus, Minnesota. The student pilot stated that he had flown for approximately 30 minutes when he lost his position. He began a descent in order to read the name of a town on a water tower when the engine began "vibrating." He then applied carburetor heat and richened the mixture, and the engine "got normal" for a few seconds. He saw that his altitude was now 2,200 feet so he turned the carburetor heat off and the engine began vibrating again accompanied by a loss in engine speed. He reapplied carburetor heat but there was no significant change. The airplane was now at 1,800-1,900 feet and the airplane was vibrating and "slowly" losing engine speed. He then performed a landing on a field instead of a road and impacted trees. He stated that he had "very little" power to clear the trees. The airplane was powered by a Continental O-300A engine, serial number 10568-D-5-A, which was last inspected during a 100-hour inspection on June 16, 2007, and accumulated 720 hours since major overhaul. Examination of the engine revealed evidence of exhaust gas emanating from a hole that was located above the last fin on the upper portion of the cylinder barrel and penetrated into the 11 o'clock valve cover screw. The serviceable tag associated with the cylinder assembly, dated March 19, 2004, identified the manufacturer as Superior. No other anomalies were noted with the engine.