Spinner, if you don’t recall, was a Toa of Air who was left injured in a deep pit by his team, and joined the Dark Hunters. Due to how heavily injured he was, he is almost completely rebuilt. He does not currently wear a mask, which indicates he no longer needs one.
Now obviously, he doesn’t wear a mask because the fan-model didn’t have a mask, but can he still wear one? Can he still use one?
I would guess no. If he was that heavily rebuilt, it is possible the systems that would normally interact with the mask are no longer functional (thus explaining why he is able to be active without one on).
So, for clarity’s sake, that IS a modification that can be made to mask-dependent species as a rule?
Anything can be rebuilt to do pretty much anything. It’s just a matter of knowledge, resources and what you do with it.
The MU inhabitants were designed, remember?