If I'm seeing the display panel pic right, you've got moisture trapped behind the front plastic?  That might not bode well for internal corrosion.  That, plus possible damage to the controller, would lead me to just replace both with either the EBR or Area13 35A controller upgrade kit to get new parts, a lot more pulling power and more features.  I assume a version is available for the Wagon, but not certain of that.  Unless something has changed, they're identical except for logos, so buy the cheapest available at the moment.  There are good vids and Chinglish documentation available for the parameter settings, here's a little info I posted recently:  https://www.radowners.com/index.php/topic,9086.msg21144.html#msg21144

I would do some research about graffiti removal. There are cleaners/solvents out there which are made for this kind of paint removal. I think the bikes are powder coated originally and the thugs probably used rattle cans to paint the bike.
The other more expensive option would be to strip the frame down completely, have it media blasted and powder coated in you personal color.
As for testing the motor I would look for a replacement battery and holder and see if you can get this back together. The cheapest option might be to find another donor ebike.
Good luck.