I think a steam cleaner would be useful right now, I wouldn't give up on the batteries either, I would suggest that they are take them off the power unit and check them over electrical wise, if no water got in to them, then a long slow change may just restore them back to life. It may be worth contacting a battery manufacture to ask for help in carrying out any refurbishment's too them, if they are lead acid then, it might be possible to replace the old sulphuric acid and distilled water with a fresh supply (just as long as the batteries have not dried out).
This may not be the long term result, but trying to buy or replace the number of these size batteries now at this moment in time may just buy you the time to get things back up and running next year.
Of course all the above may be a waste of time if the Batteries themselves have become damaged or have shorted out across the terminals.
Someone that might be able to help is the GWR company as they took over vivarail who start to build new Battery powered Railcars
Good luck