The break looks pretty clean to me, judging from the picture. Since 80% of the wood is still integral with that big block thing, I'd be hesitant to cut off the remaining wood.
If it were mine, I'd just "freshen up the edges" of the fracture, fashion a bit of wood to fit, and stain it a nice contrasting colour! If it's an old instrument, dings and dents and cracks and repairs are just part of her life story.
As far as wood putty goes, I've had good luck using it to (functionally) restore a broken old oboe. One of the ancient block-pivot kinds. One of the blocks that held the key in place had been broken off. A glob of putty, a good bit of sanding and some dark stain at least allow the instrument to function properly, and from a distance the repair isn't terribly apparent.
Thanks. I've recently had a poor experience with water-based wood filler. It doesn't dry hard enough, as does petroleum based filler. I noticed this when I had to fill a misplaced hole for a fret marker. I filled it in with the water based stuff, and after it dried, tried to reposition the depression for the inlay and the drill slid off the harder wood and into the softer filler. Did this twice before discovering the problem was the porousness of the filler. What do you use?
updated by @phroedrick: 09/21/19 09:51:19PM