I know someone has made that Mandalorian NPC model helmet-less meaning a yellow Xarga looking NPC with an NPC head is doable, however, you can't unfilter a voice meaning Xarga's voice will still be filtered as if he was wearing a helmet. You'd need to find a mask in the game that would look good with your NPCs head whether it be a Steve McQueen looking head or another head, any mask that covers his mouth would work fine so anything like a Breath Mask or a Stabilizer Mask would work though something like Neural Bands that don't cover the mouth won't work.
The Generic NPC heads like the Batu Rem head don't have the ability to wear masks because they lack something called "Mask Hooks" which is code in the head model to allow Masks to be visible. Adding the required code to any head model would allow Masks to be worn.