
Nasty cracks at the base, extending through soundboard as well as backboard. No way to fix this without removing the soundboard.
3/4 inch paulownia stock, which I split using a pull saw.
The point of no return: Here I've begun removing material from neck using chisels. A secondary mortise is being fashioned, so I can introduce a maple block which will become the new tenon. The elastic provides radial pressure on the soft wood, thus discouraging cracking.
Dry-docking the neck and headstock, to illustrate the fit. The newly formed channel is ready to be fitted with the new wood.
Roughing-in the new block. I'll tweak this with the chisel until maximum surface contact is gained. Then it gets glued with dowel reinforcements.
The finished tenon. The final cut parallels the curve of the headstock, while the hole accomodates the tuning peg. Note the dowel reinforcement. 
Headstock. Contains half of the broken tenon.
Headstock after removal of the broken tenon piece (center) and the nut (right).