DURING TAKEOFF, THE PLT HEARD A BANG AS THE PROPELLER (PROP), ATTACHING PULLEY & REDUCTION DRIVE BELTS SEPARATED, WHILE THE ULTRALIGHT VEHICLE WAS CLIMBING THRU ABOUT 130 FT AGL. AT THAT TIME. THE ENG BEGAN OVERSPEEDING, THE NOSE PITCHED UP & THE ULTRALIGHT BANKED TO THE RIGHT. THE PLT WAS ABLE TO LOWER THE NOSE & START A LEFT TURN TO AVOID A POWER LINE. HOWEVER, AS THE LEFT TURN WAS STARTED, HE LOST CONTROL & THE ULTRALIGHT CRASHED IN AN OPEN FIELD. AN EXAM REVEALED THAT THE PROP SHAFT HAD FAILED FROM FATIGUE. DURING THE ACCIDENT SEQUENCE, THE TAIL BOOM, WHICH HOUSED THE ELEVATOR & RUDDER CONTROLS, HAD COME APART NEAR THE LOCATION WHERE THE PROP WAS INSTALLED. AN INVESTIGATION REVEALED THE REDUCTION MOUNT ASSEMBLY WAS RETURNED TO THE MANUFACTURER FOR OVERHAUL AFTER A PREVIOUS ACCIDENT. IT WAS NOT KNOWN WHETHER OR NOT THE PROP SHAFT WAS REPLACED OR REPAIRED AT THAT TIME. THICK WASHERS, IN PLACE OF SHIMS, HAD BEEN USED ON THE REDUCTION MOUNT ASSY TO ADJUST THE BELT TENSION. PREIMPACT TENSION COULD NOT BE DETERMINED.