First question is why? Is there a permanent problem that needs to be fixed?
Two options:
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.