I suppose you'll have to track down the trace on the circuit board that recieves 12v when the power is turned on to the relay.
Do this by means of using a volt/ohm meter, locate all the traces leading into or out of the relay, there should be only one that will indicate 12v when the power switch is in the on position, you will clean and solder a wire lead to that terminal on the power switch and one to the positive power in terminal on the power switch and terminate those leads with an on/off toggle switch of sorts i the location of your choice.
Nice old amp that is, it has the old stud mount rf transistors.