Yeah, that would be direction I would take. The 400s are great machines overall, except for the power supplies. A bad power supply can make the the green LEDs not all light (missing -+5v), all 5 green and the Fail LEDs to light, or cause just the red Fail LED to light. You want 5 green LEDs. I've had mine for almost 20 years and never had anything go bad except the power.
If you're new to these WMS machines, just know parts are really becoming scarce. If you see any parts for sale anywhere, GRAB THEM! Backplanes, I/O boards, CPU boards, (good) power supplies, game kits (you never know if you'll become bored), and the holy grail...Dotmation screens (and related accessories...including another separate power supply). You don't have one of these in yours, but you never know if you may want a game that uses one later.
However, I have no idea where to get a hold of schematics for it. There is a link to the service manual which is full of info:
https://manualzz.com/doc/6279328/wms-400-slot-manual