Are you referring to the magazine cut-off?  The cut-off is easily obtained at gun shows, S&S Firearms, or on eBay.  This part should cost around $15 and is easily installed.  You want to check a reference like Poyer's to get the right variant for your model Krag (there are several different types).  Old NRA assembly/disassembly manuals show the technique for removing and installing.

The Magazine Cutoff (shaft) needs to be present in the slot machined for it in the Receiver for the Krag action to feed cartridges smoothly.  The later style cutoffs will work on earlier actions.  The cutoff lever is in the "up" position for the action to feed cartridges.