In case not obvious, the loose parts are from the adjustment potentiometer - similar to the green knobbed component on the right. I'm guessing it's a Humpty-Dumpty situation and you can't put it together again...but who knows! That component is inexpensive - maybe 50 cents - and can be replaced but would require some sleuthing to find a replacement and then soldering which may not be in your comfort zone.
It's curious you say you now have an IRS-1. I was looking on the web and apparently that company made an IRS-1 with only a single occupancy sensor! Here's a current eBay listing for an IRS-1 but the instruction sheet suggests it's an IRS-2. If you can find the instruction sheet that would be handy. Or, I suppose you could play Lookie-Lou and ask the seller to post photos of the complete instructions under the guise that you want to see how difficult it would be to hook it up.
BTW, the address on the instruction sheet is a residential address in Arizona which sold so it's not clear if this business is still in operation; I also saw some references to a "train-tronics" with similar products though I suppose it could be a d.b.a.
But in reading what little information there is on the web, I believe this gadget essentially behaves like your Lionel 153 IR...but has 2 detectors which, for example, would be placed on either side of a crossing to activate gates/flashers with a train approaching from either direction. Like the 153IR I believe the 2 adjustments set the sensitivity of the detectors and the time-delay. Hence, I believe you need both of the adjustments operable.
Also, what transformer throttle(s) are you using with this trolley line? And what, if any, other power is readily available (e.g., Accessory AC)?