Any 7/8″ lock will work.
Coin door locks are a 7/8″ lock that installs easily with a 7/8″ deep-wall socket, or a 7/8″ wrench. I strongly recommend installing a lock in all coin doors.
Pinball Resource sells their 1382 lock. That’s a great lock (when it’s in stock). Marco sells a different, good lock. Pinball Life has a nice double-bitted lock. The important thing about a “standard” lock, is that you can get a replacement key if (when) you lose it. The other useful property is that the lock should work smoothly.
A lock will keep kids’ hands out of games, and away from various high voltages.
If you have multiple games, key them alike. It saves trouble of trying to find the right key. Conveniently, all of the links above are for key-alike locks. Pick a parts seller and get all your locks there. (I don’t re-key heads or back doors; just keep the extra keys in the game on the hook on or near the coin door intended for that purpose.)
If you have only one game, and you get a lock that comes with two keys, screw one key into the bottom of the pinball, or perhaps the top of an arcade cabinet.
If you lose all the keys, and you can’t pick a lock, you will have to drill the lock. The goal is to get a drill bit down the center of the lock and ruin the screw at the other end so the cam falls off and the door can be opened. Then vacuum up the mess, put a new $5 lock on the game, and screw a duplicate key into the cabinet for next time.
Other locks: 5/8″ and 1 1/8″ locks
Pinball head locks tend to be 5/8 (’90s Williams, Bally, and all Stern/Data East/Sega). These have a nut on the outside to hold the cam on. (Older games took other lock types; 7/8″ is common.)
Back doors tend to be 1 1/8″ or 7/8″ depending on the exact back door.
I don’t recommend replacing these unless they’re missing, or you have no key. In any case, store the key in the game on the hook on, or just inside, the coin door.