First question is why? Is there a permanent problem that needs to be fixed?
1. Buy and install a replacement button. This needs to be only a normally-open temporary contact switch that closes only when pushed. Not high current, either.
2. You already have one of those switches on your front panel. The Reset pushbutton is the same type. So you can disconnect the wires from your on/off button at the mobo and move over to those pins the two wires from your Reset button. Then the Reset becomes power on/off, and you do not have a Reset button to use.
Somehow, one of my front panel connectors for a usb 3.0 port cooked itself, melting it's insulation and basically ruining the front panel power and reset button wire.
And, I did what you said and got a replacement kit with what I needed.