ACCORDING TO WITNESSES, THE ULTRALIGHT WAS IN LEVEL FLT AT APRX 150 TO 200 FT AGL WHEN THEY HEARD A LOUD BANG. SHORTLY THEREAFTER, THE ENG NOISE CEASED & THE VEHICLE ENTERED A SPIRALING DESCENT & CRASHED. A RIP IN THE WING CENTER SECTION WAS SEEN PRIOR TO IMPACT. AN EXAM REVEALED THAT THE LARGE REDUCTION ASSEMBLY, DRIVE PULLEY HAD SEPARATED FROM THE DRIVE SHAFT. 2 OF 4 CONNECTING BOLTS WERE FOUND FRACTURED & THE OTHER 2 HAD SEPARATED WITHOUT FRACTURING. THERE WAS EVIDENCE OF LOOSENESS & FRETTING PRIOR TO THE BOLT FAILURES. RED PAINT HAD TRANSFERRED FROM THE PROP TIPS TO THE WING STRUTS. THEPROP TIPS HAD SEPARATED & THE WING FABRIC HAD TORN AT THE WING ROOTS ON EACH SIDE OF THE CENTER SECTION. THE FAILED & MISSING BOLTS HAD BEEN INSTALLED DURING A DRIVE SHAFT PULLEY ADAPTER HUB RETROFIT, APRX 3 FLT HRS PRIOR TO THE ACCIDENT,BEFORE THE PLT PURCHASED THE ULTRALIGHT FROM A PREVIOUS OWNER. NO WASHERS, TORQUE VALUES OR SAFETYING REQUIREMENTS WERE SPECIFIED FOR THE BOLTS IN THE RETROFIT INSTRUCTIONS & THE BOLTS HAD NOT BEEN SAFETY WIRED.