There are lots of Fiat 600D cylinder heads in Ebay.it, but all are described as, "used", rather than, "reconditioned". I prefer the latter.
I've sent emails to Middle Barton, Axel Gerstl, Ricambio Int'l, Ricambio FD, Mr Fiat and Motobambino for a reconditioned one. Hopefully, one of them will have one, and sell it without the requirement of a trade-in of the old one. I effectively have nothing to trade-in.