It's easy enough to get the horns back, but the whole head is a different matter as it contains the brain which is Central Nervous System material so has to be incinerated. I would love to have the head of the chap on the left, my avatar, when he finally shuffles off - he will go of natural causes but I think it might still be difficult to keep the skull. Mounted on the wall, I think he would look magnificent
I think you could perhaps get back the skull with a special licence, if you have a genuine reason such as research, but I suspect that even then it would have to be incinerated eventually.
If you find out more and do get a skull back, please let us know how.