You know the year it was made, but not the Model...maybe a Nylon 10 or 11 or 12...76 or 77? Should be two big screws holding the sight on. Williams or Marbles may have a sight that fits the barrel contour, though drilling and tapping will probably be required. Brownells should have the sights in stock.
You can narrow the sight yourself if you have calipers, measure the width of the sight then remove the same amount of material from each side. This way you will have a sight that works for you.
If a narrower front sight is used with the current rear sight, there will be a larger gap showing on the sights. This makes it a bit more difficult to keep the sights aligned for accurate shooting.