Yes you can replace only the sight aperature itself. For the life of me I cannot remember the proper name of the part, but it is only the aperature.
The hooded sight is available from
www.fultonarmory.com I am sure.
To remove old aperature, you hold elevation knob and unscrew the slotted nut in center of windage knob. Then unscrew the windage knob (all threads are right handed threads). This will allow you to pull out the elevation knob/spindle. Once elevation knob is removed, the aperature can be pulled out.
Before you put the new one in, I like to grind off the flat boss at bottom end of the gear (order your hooded sight and you will see the up stop boss). This is the "up-stop" (you'll never raise your rear sight that high unless you shoot a looooong ways) and by removing it, you can change out rear aperatures should you ever wish to do so without disassembing sight. The replacement aperature, of course, would have to have the boss ground off on it, as well.
When re-assembling the sight, insert spindle, screw windage knob in until sight base is at mechanical zero, start and tighten the spanner nut
just enough so that you cannot push the aperature down with your thumb when fully elevated . Overtightening this spindle makes windage hard
very hard to turn.
Regards,
hps