They are no longer in business, so finding a replacement case is not going to be doable. Do you have the old case?
The bike used 2 12V10Ah lead-acid batts ... those are trivially easy to find.
A better solution would be to get 24V of newer chemistry batts and fit them into the existing bike.
You might be able to talk goodrum into making and selling you a 24V A123 pack. Or you could buy a 24V LiFePO4 pack off of Ebay. ( How much do you want to spend? )
Could you post digital pictures of the battery connectors, and the old case if you have it, and the spot on the bike where the case fits?