The pilot stated that he moved the airplane before his preflight inspection, which included placing it in a nose-high position several times. During his preflight inspection, he checked both wing fuel tanks and gascolator for contaminants. Water was detected and removed from the left wing fuel tank but no water was found in the right fuel tank or gascolator. The flight departed and approximately 20 minutes into the flight, the engine experienced a loss of rpm. He added full throttle then applied carburetor heat. The engine rpm increased slightly after he applied carburetor heat but the engine rpm returned to the previous setting. He leaned the fuel/air ratio, but this did not restore engine power. He then moved the fuel selector valve from the "both" position, to the "left" and "right" positions; this did not restore engine power. Knowing the area, he maneuvered for a forced landing on a fairway at a nearby golf course, but pulled up to avoid golfers who had entered the fairway. As a result, the airplane landed longer than anticipated and while nearly slowed, the nose landing gear contacted a curb. The nose landing gear fork fractured and the left wing then contacted the ground. He exited the airplane. Inspection of the fuel system by an FAA inspector revealed the gascolator contained approximately 12 ounces of fuel and water; about 1/2 was water. The carburetor screen was checked and found to be clean. Examination of the fuel filler caps revealed the seals were "cracked." Examination of the engine revealed no evidence of preimpact mechanical failure or malfunction.