There's not a whole heck of a lot you can adjust in a monochrome monitor unless the tube is so weak you need to do a brightness/contrast boost.
If you're pulling it apart I'd just apply epoxy over the crack in the inside of the bezel and call it good. My preference is JB Weld.
Anything you apply to the exterior will be noticeable and frankly, that's a tiny crack and wouldn't care for it. It won't get larger or become a serious problem unless you really abuse the monitor.