You need to cut the positive wire going to the power button, then put a momentary pushbutton between that positive and any ground. This shows the wire you need to cut:

Cutting that wire will remove the original button from the circuit.
Take the end connected to the motherboard, and run it to a new button.
Run a wire from the other contact on the button to any ground (under a screw).
Install the new button anywhere you want or wherever it looks good.