The batteries that were inside my battery case were 3 Long WP12-12 12v 12ah batteries. I ended up buying a set of 3 Casil CA12120 12V 12AH batteries off of ebay for $60 total that were an exact match for the old batteries. Proceed at your own risk from this point on.
Now the hard part is when you separate the two halves of the case be sure to note what cables go where or you will have a bad time getting this back together and could possibly pinch a wire in the process. Some of the wires sit in slits to keep the wires from being pinched by the heavy batteries. You may need to you a needle nose or screw driver to remove the plug from the terminals. The batteries with be taped together.
Copy the wiring and battery configuration with the new batteries. Be sure to tape the batteries together to minimized battery movement. Once everything is in place screw the case back together and give it a try. You may have to do a complete charge before you use it to see battery life similar to the original battery life.