I've never worked on a Valley machine, but in the English Mark Darts the power supply is a super super simple linear one. It's just 5 and 12 volts, the little monitor runs off 12v. Not real high current either. You should definitely be able to use a switcher, but you might get noise in the picture on the monitor. That is, assuming, this game even has a monitor - some pictures of Valley machines I've seen online just have LED displays.
What's wrong with the original supply? Might be easier to just fix it than to wire up a switcher...
-Ian