@mkp132 have you looked into running a mausberry switch? I have a Pi3b+ setup and running in and old Sears Super Video Arcade shell as the case. I installed the mausberry circuit and used the original console switch as the on/off. All you need to do is install the script and that should work.
You can even wire the mausberry off of the original cabinet power switch.
It might be easier and save you time to do it that way