PRIOR TO FLT THE SPARK PLUGS ON BOTH ENGS WERE REPLACED DUE TO MINOR ENG VIBRATION & THE LANDING GEAR RETRACT SYSTEM WIRING REPAIRED DUE TO THE GEAR NOT RETRACTING ELECTRICALLY ON A PRIOR OCCASION. AFTER TAKEOFF THE GEAR WOULD NOT RETRACT ELECTRICALLY & THE PASSENGER HAD TO CRANK UP THE GEAR MANUALLY. AFTER THE PARACHUTE JUMP THE PLT SHUT DOWN THE LEFT ENG TO ISOLATE THE VIBRATION. HE THEN STARTED THE LEFT ENG & FEATHERED THE RIGHT ENG. UNABLE TO UNFEATHER THE RIGHTENG HE ELECTED TO LAND WITH FULL FLAPS & GEAR DOWN AFTER A STRAIGHT-IN APPROACH. AS HE APPROACHED THE THRESHOLD A C-150 TAXIED ONTO THE RWY. THE PLT EXECUTED A GO-AROUND WHILE TRYING TO RETRACT THE FLAPS & GEAR ELECTRICALLY WITHOUT RESULTS.THE PAX WAS ATTEMPTING TO RETRACT THE GEAR MANUALLY WHEN THE ACFT STRUCK A TREE. THE PLT STATED HE DID NOT MAKE HIS LANDING INTENTIONS KNOWN ON UNICOM DURING THE APPROACH. NO PRE-IMPACT DESCREPANCIES WERE FOUND TO PRECLUDE NORMAL PROP OPERATION. NORMAL ELECT POWER IS HALVED WITH ONE GENERATION INOPERATIVE.