Sorry if you don't have the machine any more but I hope this helps.
You can't just "change out the power supply" on these old Atari games. They have a giant transformer in the bottom that supplies voltage to another pcb that regulates power and audio. It's not like a regular JAMMA game that can take a generic switcher.
Start by unplugging the monitor from the game board. If the monitor looks the same when you power up then the problem is probably in the monitor chassis. If the monitor is black/off looking then the problem exists in the game board.
To me, the monitor image looks like a problem with the board and/or sync.
No matter what you should replace the big blue cap in the bottom if it's not been replaced in recent years. If the monitor has the issue try a cap kit first to see if that fixes it. If it's in the game pcb you could attempt a repair or send the board to someone.