OP550A will sorta work, but it's designed for close-up work. An LTR-3208E which is in a T 1 3/4 case may work better for this. They are both in the 940nm range, you have to match the detector to the emitter LED's.
Afraid a power transformer may not work very well. But if you can find an old transistor radio or walkie-talkie or something that runs off a 9V battery, there's a good chance you can find that impedance-matching transformer in there. It will be connected to the speaker.