This is a REALLY good video about arcade monitors. If you have any doubts about doing the work, HIRE SOMEONE!

But what you want to look for FIRST is if there are already white marks on the rings that might not line up any more. If they are there but not all in a line, you should put them back so that the white marks all connect. And that marking is usually placed at 12 o'clock.

This might correct the majority of the error.

Op is in Canada. What we refer to as a 25" CRT is considered 26" in Canada (measured across the entire diagonal surface of the tube, not the viewable area). It wasn't uncommon to see CRT T.V. boxes marked 25" (26" CA) back in the day.

Any silicone is fine. Your choice of color. They used both white and black back in the day (clear is fine too). You are not looking to weather caulk a window or anything. Just placing a flexible bead across it. For the cost of a tube, I wouldn't even bother with it unless I had some laying around already.

Degaussing is normally only necessary if the corners are badly discolored. And I mean a wash of the wrong color or a rainbow etc. Not just that the three color guns don't line up there (that's convergence and purity). This distortion is often introduced by powering up the game facing one way, and then turning it to face another direction. My normal suggestion to the people that work our arcade floor is to: Turn the game off, let it sit 10 minutes, move it to where you want, let it sit 10 minutes, and then turn it on. This allows some of the magnetism to diminish and it also allows the Auto-Degaussing circuit to cool off enough to provide another "Zap" to the coil upon next power up. You can also rotate the cabinet watching the picture to see if there is a direction where everything looks the best. Then, leave it there and follow the instructions above to move it how you desire it to stay.

Of course, the metal chassis can also be magnetized and sometimes the only way is with a professional degauss coil. Its rather hard to clear this kind of distortion by just using an Electric AC drill motor (a home brew degaussing coil). If you do have an actual degaussing coil, then go to town on it. If you haven't seen instructions elsewhere for using it, the technique is to power it up flat to the screen. Roll it around the screen in a swirling motion trying to cover all the territory. Ending by moving it to center. Back off with it still flat to the tube (and still powered). Once you get about 10 feet away rotate the coil to be perpendicular to the screen (90 degree rotation), and then turn it off.

My trick for degaussing an arcade monitor is to take a CRT PC monitor, put it face to face with the arcade one (while they are both on) and hit the degauss button on the PC monitor. Works incredibly well.

The panels that light up are usually burnt out and are not cheap to replace. Most people put a single fluorescent tube up there.

I had recently read that people are getting smaller UL's and making their own to fit the rectangles. Supposed to be a fraction of the price. I cannot verify this since all of mine have always been replaced with a florescent. I'd be curious to see if something could be done with LED strips.