After liftoff, the airplane appeared to stop climbing at about 90-100 ft AGL; its wings rocked side-to-side, and it then pitched nose down into the ground. The engine was heard until impact. Mixture control found in full rich position; density altitude approximately 5,000 ft. Oil stain found near prop governor; AD 90-04-06 and Lycoming SB No. 488, prop governor line & elbow fitting, NCW. Engine teardown showed prop governor screen blocked with numerous contaminants; metallic flakes on oil filter; carb body dirty; carb float attach pin worn; plugs showed rich fuel condition; mag timing 3 deg off; intake/exhaust rocker arms improperly installed; piston rings worn beyond life limits; No.2 cyl top ring broken; numerous rod caps & bearings installed backwards; all crank pins worn beyond service limits; all main bearings worn beyond service limits. Serial number on data plate did not match case number of the engine. Airplane was purchased 1/23/90 by the pilot.