I already played with Pager before (if you search for POCSAG or Pager here will find some posts), but it is a rabbit hole, every time you search for it will discover many useful things. So I decided to list some useful links here:

Pager for HAM guys: https://www.qsl.net/kb9mwr/projects/pager/plan.html

Details about Motorola Advisor board and crystal: https://goughlui.com/2016/01/15/tech-flashback-motorola-advisor-pocsag-5121200bps-pager/

MotoLoRa, this guy used the Pager Advisor as a screen to a LoRa communication: https://hackaday.io/project/183267-motolora-advisor/details

FLEX protocol implementation: https://github.com/dl1com/flex

Another page with FLEX protocol info and implementation: https://www.discriminator.nl/pdw/index-en.html

Phillip Johnstop from Emdedded Artistry post about debugging FLEX: https://embeddedartistry.com/blog/2019/12/02/a-case-study-on-finding-fixing-and-testing-a-firmware-bug-without-hardware/