Just for fun I checked the old battery voltage and it was 0.2v (a far cry from 3.0v). The amazing thing is that the DX7 memories were still working perfectly! The original Sanyo battery showed no signs of leaking, and the only hint of trouble was the battery voltage (on the DX7) was reading 4.5v.
After I reassembled the keyboard I reloaded my favourite sound bank from the computer, and it is up and running again. The only thing to need resetting was the Function data.