From the looks of the schematic none of the parameters for the part are too critical. You just need to find an optocoupler in the same package (6-pin DIP, 0.300" lead spacing) with the same pinout. That's a pretty standard pinout.
Any of the following from the "4N" series optocouplers should work: 4N25 thru 2N28 or 4N35 thru 4N37. These are very common.