Bells are fairly universal, and sensors and cables can work with any brand of replacement.
Personally, I would replace with a Scantronic 9651 as it comes with full instructions and you get a full LCD screen; others might recommend something similar like a Pyronix Euro etc...
However. most important, before you do anything, is to test each cable to each device (inc. the bell) individually to rule out damage, shorts, or opens. Your son should be more than
capable of this as it's pretty much the same process as testing any cable and not just an alarm thing.
Personally, I would also take the opportunity to upgrade the PIR sensors at the same time, but let us know what the cables are like first!