Greenstuff/Kneadite is available fro GW stores or from Northstar, and many other retailers those just spring to mind.
I prefer a putty called 'Procreate' which comes in either grey or brown.
For the horns i'd drill the head and implant a suitably bent piece of steel wire (I find the brass stuff doesn't take paint/putty as well). Then use a thin layer of putty to provide shape and bring to a point.
If you're not confident and can wait a little while I would be happy to have a go at this conversion for you (as the project is at least partially my fault
