Bruno, your best chance would be to contact Ferrari UK. They would most probably have one (or more) in stock, but be warned: It'll be expensive!
http://www.ferrariownersclub.co.uk/ferrariParts/index.asp
Why do you have to replace the head, if you don't mind me asking?
http://www.ferrariownersclub.co.uk/ferrariParts/index.asp
Why do you have to replace the head, if you don't mind me asking?