It all seems a bit over-complicated and could easily be achieved using simple OR diodes as follows :
Throw away the transistor idea.
Battery -ve connects to gen -ve and LED cathodes
Generator +ve connects to battery +ve via a blocking diode (D1)
Generator +ve connects to LED switch via diode (D2)
Battery +ve connects to LED switch via diode (D3)
Whichever is the greater of generator versus battery feeds the LED switch and hence the LEDs
edit : Darn it beaten to it whilst I was typing up my suggestion