I am not optimistic about repairing the head itself. Even if you match the material exactly it still amounts to a crack which will raise the resonant frequency of the circuit.

The piece will probably break off when you polish it, and you better polish it or it will probably tear up every tape you put in it.

Your little piece of ferrite is going to have to be firmly stuck in the back, and also have a tiny gap on each side.

Good luck, you will definitely need it.