> May be good to post the part you found if more available in case others
> have the same problem.
The part I got was a Honeywell SDP8600-003. The SDP8601-xxx is an preferable substitute - the pins should be easier to fit into the board. The SDP8602 is electrically compatible, but the pins are too far apart so you might have to jury-rig the wiring.
I had a hard time cramming the SDP8600 down far enough, so that it was lined up with the infrared LED. But it's absolutely vital that the sensor be aligned with the emitter. The light is shining through a narrow slit.
The last three digits in the the part number indicate the sensitivity. It probably doesn't matter what you get.
Now the bad news: these parts are no longer manufactured. I think they went obsolete in 2019. Finding someone who's selling them is tough. There are similar parts numbered SDP861X, and they're still in production, but their output signal is inverted. You'd have to jury-rig a NOT gate into the circuit.