This should be your customized unifi.py:
custom_components/device_tracker/unifi.py
This should be a copy taken from homeassistant/components/device_tracker/__init__.py:
custom_components/device_tracker/__init__.py
Be advised that if your customized version of unifi.py is based on source code from a previous release of Home Assistant, it may fail. 0.89’s version of unifi.py may contain modifications that make your customized, older version incompatible.
This is precisely what happened to me. I had customized versions of MQTT climate.py and alarm_control_panel.py based on code from 0.88.1. Climate worked but alarm_control_panel did not. It had undergone a small but important modification in 0.89 that prevented my, ever so slightly older code (0.88.1) from working.
My “Great Migration” adventure is documented in this post. 