Assuming it is a transistor, it will probably be a generic type - should be able to determine NPN or PNP from the circuit - my guess would be NPN connected as a common-emitter switch, assuming LED common is positive. Depending on the load current, somethiong like a BC548 would probably do. Just check the pinout - could be centre-collector or centre-base - collector probably connects to the LEDs.
Looking at the PCB pic I;d guess centre base, so BC548 would be a good start