Probably, the best Scope option is a 'rail', that places a long eye-relief Scope out on the barrel, 'Scout Rifle' style.
This barrel mount uses the rear-sight holes and requires the hand-guard and rear-sight be left off the rifle.

A vendor, "Kraghaus", listed in KCA Classifieds, makes and sells this mount.

The S&K company makes a 'no-drill' scope-mount for the U.S. Krag. This mount is off-set to the left and requires the removal of some stock wood.

Receiver mounted scopes on the Krag have to be 'off-set' to allow ejection. Also, some type of 'cheek-pad' is needed or you soon get a stiff neck.
There may also be interference with using the issue 'safety'.

BTW - There are lots of Krags, often cheap, that have been 'drilled and tapped' for a scope. Sometimes an extra, 'special purpose', Krag is a good idea!

Attached photo - A model 1898 carbine ruined by scope holes, but, still a useful 'shooter'.

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