I presume the optoisolator board you mention is the one on the lenze controller, if so leave it on and use it, as it isolates the 0-10v spindle speed from your control board to the speed controller, a safety circuit.
If you have a look at the wiring diagram, either in the manual or the one i posted on tudder thread, you can trace it back, control will be from the 0-10v spindle speed output from your new controller, and a switched 0v to the relay, The relay will have a default position when lathe powered this is normally reverse as turreted lathes machine from the back, but if your using a toolplate or quick change tool holder the you can run it either way as default, to change the default direction, simply swop the motor wires over in the relay.

The photo interrupters are wired together so you have +ve ,-ve ,sig a and sig b . from memory if you trace the connector on the lower board the +ve voltage comes from a gert long resistor, you should be able to trace it back and mimic the supply.

The whole speed control direction circuit seem to be a bit of a nightmare at first glance ,but is easy to understand one you work it out.

Will post a bit more was going to say tomorrow but suddenly realised it will still be today, lack of sleep messes with the days ;)