Yours sounds almost exactly like mine when I got it, I think it had 47 hours. Most any battery that will fit should work. I actually have a 24 volt nicad battery from an airplane that I ruined which works fine on mine now (actually that is in my 804b- regular batteries are in the 804a). Hopefully, the cables are still there. Have you run military style generators before? Have you downloaded the books and read them? The Isuzu engine is different than other 002, 003, 802, 803, or 804b engines in that there is not an electric primary pump, it is mechanical. That means when you turn the main swith to prime and run, you will not hear anything. There are several things to check before you start it and there is lots of knowledge here to help. There are also some modifications that will need to be done or check for to keep from killing your new generator (search fuse mod). You will also need to hold the switch in start until the oil pressure stabilizes, not normal for most all other engines I ever ran. It feels weird at first but is designed this way and will not hurt the starter. If you bought this to run 3 phase equipment, fine. If for single phase, not so much since it will not do 120/240 single phase (not designed for) as everything up to the 804 size was. I am not saying it is impossible, but if you bought this for your house, you may want to rethink it for something more suitable. Please help us help you, details of your experience and plans for it will get better answers. Pictures are always welcome.
Robert