At
https://www.electronicspoint.com/projectlogs/low-drain-long-life-audio-clicker.163/
you'll see a fairly simple build which will hopefully do the job.
At a 1-per-second 'click' rate, rather than a continuous tone, the circuit should run for about 1-2 years.
As output pulses are low voltage, the clicks are quite soft, but pets should hear it ok.
See if you can persuade an electronics family member or friend to build you two of these, for food dish and loo.
Or have them pro-built for kitty.
Frequency (audio click rate) or different housing size / shape can let the cat know (by carrying / training first) which "audio beacon" to use when.
Add:
Just thought of something else -
Cats have excellent smell sense, probably enhanced if sight goes.
What about varied flavor scented fridge-magnets / cardboard 'smellies' stuck here and there throughout the house?