I don't know anything about these cards so these are just generic suggestions:
Is there some other image of the original unmodified firmware you can program onto the same EPROM? That would help confirm the problem is the chip and not some other issue with the firmware you're trying to use. Although to put the certainty at 100% you'd ideally want to copy your own firmware from your own card, and you couldn't do that without knowing the chip type when you dump it.
By testing continuity with a multimeter, can you figure out which pins of the chip are connected to GND and which are connected to +5V (or even +12V)? That might add some clues to guess what pinout it's using.
You can probably find an ISA connector pinout online that will tell which pins of the connector carry GND and all voltages. You can check continuity between those locations and the pins of the ROM chip you're trying to identify.
Instead of tracing the continuity, another way to get the same info would be to power it up and measure voltage at the IC pins (or at the empty socket) vs a known ground (case of the PC), but then you have to be careful not to short anything. It's safer to wrap the probe with tape so only the tip is exposed. Anything that reads a steady voltage that makes any sense is probably tied to that voltage level and not switching.
Then compare with some EPROM datasheets and see what makes sense.
