A web search doesn't show any supplier of 850 spider wiring harness (loom) in the U.S or Europe, at least none that I can find
What is wrong with your existing harness?
I'm a great believer in repairing what you've got, if at all possible. In my experience, Fiat wiring harnesses give relatively little trouble - most problems are due to people messing with it, poor earths or loose/corroded connections.
These issues can probably be corrected more quickly than stripping out the existing harness and installing a new one......