WARNING! Know what you do, there is deadly electricity involved!
As this is a CRT the geometry is controlled by static and electric magnets around the neck of the CRT. These magnets can be mechanically and electrically adjusted until the geometry is correct (the image is centered and the scan lines run horizontal).
For adjusting the geometry of any CRT you need to work inside the open and powered on Mac!
WARNING! If you are not an experienced electrician and know how to handle high electric voltages (high enough to instantly fry you to death!) KEEP YOUR FINGERS OFF!
From your question I gather that you are not the experienced technician required for this job because if you were you would not have asked this (the diagnosis and the cure is pretty obvious to an expert).
KEEP YOUR FINGERS AWAY and let an expert do it.
Hope you can get it fixed and have a lot of fun with that nice old Mac!