The panel is faulty either way if the memory has been lost, the keypad is part of the system and are compatible with each other, you're not finding a panel that will work with that 30 year old keypad.
They are pretty resilient panels to surges and short circuits to be fair, but it's done it's time. Could diagnose the same from what you have said in OP.
Things that I wouldn't be able to tell you is if the keypad is worn on some of the digits, this may have stopped them accessing the panel for example.
Maybe their engineers were not even born when the panel was manufactured and not seen one to work on... Either way keypad is of no use so may as well be dead.
Prices will vary a lot based on location, installer, approval, kit quality etc etc.