Looks interesting from a hobbiest perspective. Good to keep old kit that works still viable.
Might want to make sure you employ ‘watchdog timers’ if you haven’t already to mitigate ESP lockup
Plenty of commercially available solutions that allow you to do this. Personally I question why you would want to rely on a 30 year old alarm, its well beyond its serviceable life (even if it is still working). For the money and effort you would be much better offer replacing it with a modern alarm.
It cost me £18, the panel works just fine which was why I chose to upgrade rather replace it and I learnt how to program in c++. Oh and it was great fun.