I have a 1.9 jtd 2003 and I'm wondering if it
should have a sump guard underneath.
If so then mine is missing... Is it metal or plastic?

My local Fiat dealer/workshop says it did not come with a 'splash guard'

They were plastic, but I think that from 2002 onwards they were done away with. They were a bit of a pain anyway, especially if (or more likely when) the captive nuts securing them came adrift.

However, the oil drain plug is a bit exposed to road humps and the like and it's sensible to be aware of that if doing a bit of rough roading!