Pictures of that rifle show a front site. If you are sure this is not the original and it is shooting low, that means the post is too high for you and could be filed down to shoot where you want it. Almost 0.2" of extra front sight height is there if it shoots 3.25" low at 25 ft and there is about a foot-and-a-half between the front and rear sights? Are you sure you can't set the rear sight up a bit? I couldn't see clearly from the picture whether it is stop-adjustable?
Nick