Based on the last update from Mike's Metal Fab on Youtube 1670 is nearing completion. The tender is done and painted. The Cab is nearing completion of metal work.
In a previous video he was showing and discussing the tender for 520 and how they were removing rust and dirt from the tank as well as looking to fabricate new supports so I would assume once 1670 is complete work would move towards getting 520 done in the next two years.