THE PLT REPORTED THAT THE ENGINE BEGAN TO RUN ROUGH & LOSE PWR WHILE HE WAS CIRCLING SOME PROPERTY HE OWNED. HE SUBSEQUENTLY ATTEMPTED TO LAND AT A NEARBY ULTRALIGHT AIRSTRIP. HE STATED THAT HE DIDN'T REMEMBER ANYTHING AFTER THAT. THE AIRSTRIP OWNER REPORTED THAT HE HEARD THE ENGINE 'POPPING' AS THE ACFT APPROACHED THE AIRSTRIP AND ENTERED THE PATTERN. HE STATED THAT THE ACFT WAS 'TOO HIGH & TOO FAST' WHEN IT REACHED THE END OF THE RWY. THE ACFT THEN STALLED AND RECOVERED TWICE AS THE PLT MADE AN APPARENT ATTEMPT TO TURN AROUND & LAND IN THE OPPOSITE DIRECTION. SHORTLY THEREAFTER, THE ACFT STALLED AGAIN & CRASHED. POST-CRASH EXAM OF THE ACFT REVEALED NO EVIDENCE OF PRE-EXISTING MECHANICAL MALFUNCTION OR FAILURE. THE ACFT WAS FUELED W/ AUTOGAS AND THE CARB WAS EQUIPPED W/A COMPOSITE FLOAT. THE ACFT WAS AUTOGAS STC'D. WX CONDITIONS RPTD AT CLT ABT 20 MILES NORTH OF THE ACCD SITE INDICATED A TEMPERATURE OF 74 DEGREES AND DEW POINT OF 62 DEGREES ABT 50 MINUTES AFTER THE ACCD.