On November 13, 2012, about 1720 eastern standard time, a Piper PA-18-150, N9764P, was substantially damaged during a landing rollout following a forced landing to a field in near Canton, Georgia. Visual meteorological conditions prevailed and no flight plan was filed for the local flight. The airline transport pilot and passenger were not injured. The personal flight was conducted under the provisions of 14 Code of Federal Regulations Part 91. The flight originated at Air Acres Airport (5GA4), Woodstock, Georgia, about 1650. According to the pilot, he and the passenger were performing flight maneuvers at 2,500 feet mean sea level (msl). After descending to 2,000 feet msl, he noted the engine rpm and airspeed began to decrease. The pilot checked the throttle position, mixture control position, fuel selector, magneto selection, and cycled the carburetor heat, but was unable to regain full engine power. He elected to land the airplane in a nearby field since the airplane only had partial engine power and descended using full flaps and a side slip maneuver. After clearing trees, the pilot landed the airplane in a grass field; however, the airplane struck a slightly raised path in the field. The airplane bounced, nosed over, and came to rest inverted. The pilot turned off the fuel selector and the battery prior to exiting the airplane. Further inspection of the engine and removal of the engine cowl revealed a hole worn in the lower section of the engine cowl. The engine cowl was reinstalled and it was noted that the carburetor heat control arm did not have much clearance from the lower engine cowl and the hole in the engine cowl was in the vicinity of the carburetor heat control arm. In addition, the carburetor heat control arm would contact and catch on the lower engine cowl when pressure was applied to the cowl. When the carburetor heat was pulled ON, the arm would catch on the cowl and limit the application of carburetor heat. According to the pilot, the lower engine cowl was fabricated to fit on the airplane. However, during a postaccident examination of the engine cowl with a Federal Aviation Administration inspector, the pilot noted that the lower engine cowl did not have adequate clearances from the engine. The most recent annual inspection was performed on September 20, 2012, and at that time, the engine had accumulated 1034.7 hours of total time in service, and 141.3 hours since overhaul. The 1715 automated weather observation at Cherokee County Airport (CNI), included wind from 300 degrees at 6 knots, visibility 10 miles, clear skies, temperature 10 degrees C, dew point -5 degrees C, and an altimeter setting of 30.27 inches of mercury. The carburetor icing probability chart from Federal Aviation Administration (FAA) Special Airworthiness Information Bulletin (SAIB): CE-09-35 Carburetor Icing Prevention, June 30, 2009, shows a probability of icing at cruise/glide power at the temperature and dew point reported around the time of the accident.