Go to a taxidermist and tell him you want your horns on a head mount, he can buy a cape to put on a form. The whole head isn't used in a head mount, just the horns and the top of the skull where they are attached. As for cost, every taxidermist has different costs. There should be no legal problems, if you are worried, just take leave the horns attached to the board when you take it to a taxidermist.