If you haven't already, you could try to replace the battery in your fob. That may or may not help.
If not, you could try and get the exact model number of your specific alarm and contact the manufacturer to see if any parts are available. I did a quick internet search and it looks as if Excalibur is still being made, but they may now be owned by Omega. I'm not 100% positive on that, it's somewhat difficult to tell. A quick call or e-mail could confirm.
All of this being said though, it may end up being more trouble and expense than its worth. You can get a basic alarm for very little money now-a-days. If you are reasonably handy and have a basic knowledge of electronics, they are not terribly difficult to install. Things about alarms that get more complicated are when multiple relays are required, remote starts, newer cars that require bypass modules, etc.