That connector, if it is like the Atari 2600 joystick connector, is a common DSUB 9-pin connector or DB9...same is a 9-pin serial port connector type. So the connector is not a problem. Reverse engineering the pinout is another problem. I can't find a service manual for that thing anywhere. If it was me the first thing I would do is pick up the phone and call Fostex parts/service and ask if them have it. You can also inquire at servicemanuals.net to see if they can get it...they don't have it in their catalog but you can inquire with them if they can get it. Outside of that its opening the unit up and seeing if the boards inside are screened/labeled with markers to identify what is carried by each pin. I suspect +12VDC, -12VDC, +5VDC, +48VDC, possibly a separate +12VDC rail for the transport motors, and then 0V rails relative to each.
Sorry I couldn't be of more help.