You're close, but no, there is no unused hole left on the board. Locate the yellow/white wire coming to the #13 pin on the MPX board. This wire supplies +24vdc that is first routed through the front panel muting switch to the board then to the LDR. Once you locate the wire, you can follow the short trace to the pad where the LDR attaches. (It's the only thing this wire connects to on the board).
Remove the old LDR. On the new LDR, you can tell the LED end by two things-- the leads are much closer together where they exit the plastic body, and there will be a polarity marking. The photocell end has wider spaced leads and there is no polarity marking. The LED end will be placed closer to the center of the board, just like the lamp was in the old LDR. You will attach the 1K resistor to the hole with the 24v supply, standing upright. Bend the leads on the new LDR so that the + or anode side of the diode will stick upwards to attach to the resistor. The other three leads go downward in the usual fashion through the board. Solder them, then hook the resistor lead to the LED lead and solder them together "in space".
The other side of the LED (the cathode end) goes to C406 & R410. R410 connects to Q403, which connects to ground. Q403 is what switches the mute drive in and out by grounding the LED/resistor circuit.
+24v ---> 1k res ---> [ ->I- ] ---> R410 ---> Q403 ---> GND