I'd see if this wouldn't work.. especially if that rifle is grooved.
Accessories - Open/target sights - PyramydAir.com
http://cdn.pyramydair.com/images/acc/PY-A-2003_Williams-Diopter-Sight_1472590140.jpg
And in my book, peep, receiver, diopter, aperture sights are all the same as they all have a round hole in them.
anyway, good luck
I've looked at that Williams before - not a bad price. I wonder though if you need a matching front sight? Seems they imply not all front sights will be able to work with that one - I'm assuming due to the height of that sight - may not match the height of the existing front sight?