Confirmation of circuit function - watchdog circuit (moot)

So I have this old piece of test equipment I want to duplicate, but with more modern circuitry...

So far I have been able to work out some design changes, but as I started laying out the circuit attached in KiCAD, I had wondered if this could be replaced with a more modern part.

[Edit: I was able to find a working subassembly in the 'boneyard' I have at hand and was able to prove this out as the watchdog circuit. Leaving the diagram up in case anyone needs the circuit]