It's become harder and harder to find any parts or kits for these games as they're almost 30 years old. Someone may have what you're looking for though. If you are a subscriber, you could post a WTB ad in the classified section. Or perhaps they'll see it here. I don't have kits, but I have internal parts I plan on posting to eBay soon (CPU, I/O, Dotmation screens, etc).
Regarding the bill validator, originally these games shipped with a DBV-145 unit. They were slow to accept bills and now are obsolete, I don't think you can upgrade the EPROM anymore to accept newer bills but I could be wrong. These machines can use a DBV-200, which are much faster and still easily available. They're officially discontinued so I don't know how recent the latest firmware is. eBay is probably best for finding these things.
The locks are just generic locks, used for slots, pinball, video games. There isn't a specific 'Williams' lock, just search for one that looks similar to what you have and it will probably work. The barrel should be 'oval' shaped, not totally round. Personally, since my game isn't used commercially, I just use keyless locks that have a knob instead of a slot for the key. Keeps the door closed, that's all I need. You just want to make sure the length of the barrel isn't too long since the lock is really close to the coin hopper. About 1" should work. You can measure yours to make sure, I eyeballed mine just now to see and estimated about 1".