WITNESS REPORTED THAT THE AIRPLANE ENGINE WAS RUNNING ROUGH DURING TAKEOFF; THE AIRPLANE SUBSEQUENTLY DISAPPEARED FROM SIGHT IN A NOSE-DOWN ATTITUDE. THE PLT LATER REPORTED THAT HE COULD NOT REMEMBER THE EVENTS THAT LED UP TO THE POWER LOSS. POST-CRASH EXAM OF THE AIRPLANE REVEALED NUMEROUS MECHANICAL DISCREPANCIES INCLUDING SEVERAL SPARK PLUGS W/CRACKED INSULATORS, AN INOP MAGNETO, FUEL SYSTEM CONTAMINATION, AND FUEL TANK VENT SYSTEM BLOCKAGE. 'VISE-GRIP' TOOL MARKINGS WERE FOUND ON SEVERAL OF THE SPARK PLUGS. THE ENGINE RAN AND DEVELOPED SATISFACTORY PWR ON A TEST-STAND AFTER THE INOP MAG AND SPARK PLUGS WERE REPLACED. THE ENGINE WOULD ONLY DEVELOP PARTIAL PWR AND WOULD NOT EXCEED 1200 RPM WHEN THE ORIGINAL SPARK PLUGS WERE REINSTALLED. THE PLT, WHO WAS ALSO THE ACFT OWNER, WAS NOT ABLE TO PROVIDE ANY MAINTENANCE RECORDS FOR THE ACFT. THE PLT'S LAST MEDICAL EXAM WAS 5 1/2 YRS EARLIER, ACCORDING TO FAA RECORDS.