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.