If you used something like an OP240 or OP245 IR Emitter then their corresponding phototransistors are OP550 or OP 560 for the OP240 and OP555 or OP565 for the OP245.
The IR Emitters have been hard to get for a while but fortunately you already have that component.
Before changing any of these components I would measure the existing signals and make sure the part is where the fault is.
-sonavor
