What happens? Does it power up at all?
Don't know these specifically, but I've built a few VPins
The power button will be connected to the soft-on pins of the motherboard, so if the issue is you have no controls, but the power button works that makes sense
In that case it's probably the interface board that converts the button inputs into keyboard presses, could be a dead IPAC (or whatever they used in these) but more likely a loose cable/missing ground on the button harness.
Looking at it, I suspect you'll need to remove the playfield screen to get at the guts of it