I'm no expert on sporterized Krags, but I believe that all of the high-grade receiver sights used the cut-off hole to some degree. The ones you really want to avoid are those which need a hole in the receiver.

I believe that at least one sight also used the sideplate screw hole, with a longer screw, but I THINK it may have required a slight cut in the wood.

Rice made a rudimentary peep which mounts non-invasively on the bolt sleeve, and a couple of makers had a sight which dovetailed into the rear of the cocking piece.

Of course, ALL Krag aftermarket sights are, I believe (but could surely be wrong!) long out of production. Gun shows will be your best source, and a fine Lyman or Redfield won't be inexpensive.

If you NEED the cut-off, a Rice might be your best bet.