I doubt very much whether it’s possible to “recover” the previous functionality, as Google has discontinued the GCM service.
It would conceivably be possible to re-write the local server source code to use the replacement FCM service, then re-compile the code into a Java executable. But, it seems unlikely that the legacy app would work with FCM, and the app source code is unavailable.
Using a 3rd party solution such as Pushover is probably the simplest solution.
Just to add to this - I signed up to pushover and asked Microsoft Copilot to do the code for me. It wrote code to put on the blynk server (in my case running on a raspberry pi). The python code monitors those pins that you specify and then sends alerts to your phone via the pushover app.
I just had to give Copilot the token keys etc for my particular setup and it wrote the lot.