To power up a single atx psu,just wire a cable on pin 14 and any ground from the 20 pin connector.
Well,you can place a switch on this wire and work it like an AT psu.So put the switch of the original psu in the front of the case,and place it on the metalic plate.
About the mobo voltage pinout,it's very easy to wire it with your new psu.Use a voltage meter to connect them right
Good luck!
