Check circuit diagram. I had a switch on my CB750 hidden under the tank for several years since the stock ign switch gave up.
I think I wired them all 4 together to switch on.
Red 12V on one side of the switch- all the other on other side
CB750 close 12V Red from 15A fuse - Black which is main power.
Plus closing brown/white - Brown wire to rear light. Brown white get 12V via a jumper to the black circuit.
This make it same thing to close Red 12V with all the other. CB500 switch can be same.
I used the wires from ign switch I had cut, soldered on a small switch.
There is also a number on Honda stock switches, maybe possible to order spare key. You must know type of key...