Break beam sensor to replace piezo transducer
This is a dead end. It's easier to design a beam sensor-triggered flasher (DC, relatively reliable currents) from scratch than it would be to convert a piezo signal (very low currents, AC) sensing circuit.

So use your break-beam sensors to trigger a flasher. There's a lot of "flasher circuits" out there, and it's also not hard to figure out how to amplify the signal from the sensor. Good fun!