This CRT if very bright and sharp, there's only a flickering on the top corners, is there any fix? What I should look for?
I'd start looking at bad solders on the flyback transformer. I had a similar problem with a Mac 14" colour monitor, and that was the culprit.
More common to the toaster Macs is bad capacitors (look for capacitors that have deposited black/brown poops on the board, or are swollen) and the two (are there two in the Classic II?) large diodes that tend to go bad. They usually have a cooked look on the board around them.
Usual cautions, yada, yada; care working around high voltage equipment.