instead of an expensive-shipping-cost ring neo magnet, there are other creative options for a rear peep, or at least something that you can trial and see how well it works for ya.
you can ...
* double stick tape a washer to the existing rear sight.
* double stick tape on a hardwood rectangle that has a washer glued to it.
* bend a piece of sheet metal or brass into an "L" shape, on the vertical side of the "L" draw a vertical centerline and drill a series of 1/16" holes,then drill a larger hole in the L's horizontal "foot" side, use the breech tang screw to hold the metal foot - there are many more ways to make this quickie fixed tang aperture rear sight.
i've also super glued a hardwood block to an existing rear sight and then either drilled an aperture hole in the wood or added an aperture disc ...
i retrofitted a lyman 57sml tang sight to several trad sidelocks (as well as the 17a globe front sight) ...
with some real creativity, and some serious thinking, you may be able to retrofit a marbles peep tang sight or some other "buffalo rifle" rear tang aperture sight (those can be really expensive - i know, i have more than a few for my sharps and rolling blocks).