I would leave it, it's an antique and the new owner is going to hang it on the wall like you did. If you file a new notch, it will change the geometry of the tumbler and the sear will go into half cock every time you pull the trigger. It looks like the original tumbler too. You can get original and repro parts, but they will need to be hand fitted. A broken half cock notch is not that uncommon for some reason. It shouldn't actually effect the value of the rifle, it's a very nice rifle, indeed.