Restore Downloads icon to Finder Sidebar
Even though what you need is very little, there seems to be no easy workaround except for the SIMBL plugin: 

How can I change Finder&#39;s sidebar icons in OS X El Capitan?

As an experiment I uploaded my downloads folder to google drive. You can try downloading it and see if it works:

https://drive.google.com/drive/folders/1U3dY4N_ZDLtnq93yoKVYVi4F5WqBaa8i?usp=sharing