OK. Let's check a few things first. With the lens off the camera and the aperture ring is set to f/22, is the lens opening at the smallest size? Does the lens aperture open up properly to fully open at f/2 when you move the pin on the back of the lens sideways toward the other end of its slot? Does the lens aperture snap back quickly to the smallest opening when you release the pin? Does the lens aperture open up fully when you rotate the aperture control ring to f/2.0? Is the mirror down in the viewing position? Is the front surface of the mirror reasonably clean? (It is a first surface mirror and it is easy to scratch it when attempting to remove things on the surface. If it is not really noticeable, I would leave it alone; it will not affect what is going onto the film.) Is the viewfinder screen in the top of the camera still fairly clean? As a comparison, can you see the lens f-stop numbers through the little rectangular window below the viewfinder screen? Is the viewfinder screen properly seated in the top of the XG-1? (There should be no obvious angular displacement.)
If you can look at these things and answer while I see if I can find my XG-1, we can get started. And, it is possible that we should be doing this on either the Minolta Group or in the Camera Repair and Modification Forum.
And, the Minolta ROKKOR 2.0/45mm lens is a simple Tessar design, and it is quite a good lens. It is often under appreciated. With all Minolta lenses, I do suggest storing them off the camera body with the lens aperture set at the smallest lens opening of f/16 or f/22. That allows the lens aperture control springs to relax and not be kept under tension while stored. Put caps on both the front and the back of the lens, and put a body cap on the XG-1 to keep the inside clean while they are stored.