I just went through a similar effort on my Ademco FA110C system from 1997.
If you can swing $200-250, best course of action would be to get a Vista 20P panel to replace your old one, along with an Envisalink module and at least one keypad.
I didn’t want to go through the effort of replacing my entire system. Looking through manuals for my system I found that it had a key switch arm/disarm option, which is basically a momentary switch that connects two 12v terminals at the panel together for around a second to toggle alarm/disarm state. I was able to simulate this keyswitch with a zwave relay. Was also able to use a contact sensor wired into one of my keypads to retrieve alarm state. Check the manual for your system and see if there’s anything like the keyswitch that you can use to arm and disarm your system.
I have some pics here, but would be happy to share more info if you want.