I'm not familiar with Space Invaders specifically (the monitor type), but have you simply tried adjusting some of the pots on the monitor chassis? There should be several knobs located on that board that control various aspects of the image.
1 should control the horizontal size of the image, and 1 should control the vertical size. Adjust them until you've got it perfect. But if this doesn't work, I probably won't be of much help. I'm a novice at repairing electronics, but simple adjustments are not hard to do.
Also, be very careful when adjusting those pots. You don't want to get a nasty shock from touching another part of the monitor.