I would try the two (good, currently parallel) 6V batteries you have and nothing more. If that's sufficient capacity for your needs, then you're all set to go and you don't need anything more. That should be basically the same capacity you had with the three 6V batteries initially, by my thinking, and quite possibly more than you currently have with the one cooked battery included.
If you want to spend lots of $$$, landyacht's advice is probably about the best you'll be able to do.
If you want to spend lots and lots and lots of $$$$$$$, you could probably find some way to cobble together a Li-ion battery bank (and put in the requisite chargers and control circuitry) that would have a much higher capacity.
