Since you have the schematic, it's trivial to wire up an adapter to read the chip in something like a TL866 or any other EPROM reader/burner. All you need to do is compare the pinout to something standard like a 2716, most of the pins should all line up 1:1, the chip select line can be tied to Vcc or Gnd depending on what it needs. You can either measure what it is in system or you can tie it to one, try reading the chip and if you get all FF's tie it to the other.