Normally the key to the back door or head is on the back door key
hook on the inside of the coin door or just inside the door to the left
or right. If the key is no longer there, check the bottom of the main
cabinet, as it may have just fallen off the hook. If the game was stored
with the head removed the key may be at the very back end of the main
cabinet, usually in a pile of debris.
If you still cannot find the key, but the game was in storage with the head
removed you may be able to reach up inside the head with a wrench or
screwdriver and remove the latch.
As a last resort before drilling out the lock, try all your keys that might
fit the lock. A surprising number of keys will sometimes open these locks.