Well, I picked up this manga on a whim, and I've actually been following it for a long time, If the previous translator ever comes back, I'll step aside
I would say, because of the people arguing here about whether it is proper or not: try to contact the previous (non-MTL) team and ask if you can take over in the capacity you feel you can do.
So you at least can have the peace of mind that you aren't making their work-in-progress feel redundant to them, and worst case scenario (which I imagine is the fear and motivation for taking a "stronger" stance against this) stop working on this definitively.
If they haven't answered after you've tried consistently for a week or two, I'd find that as enough due diligence to continue. Unless IRL stuff is eating them alive, at least a short response should be possible within that long a window.