In the meantime I implemented another part of the damage feature. The game shows the damage indicators you proposed above in the right part of the screen. If you crash trees and other world objects or flip the car over, the indicators will go from green to yellow to red. You can use the patch in the attached ZIP-file for testing. You also have to put the three PNG files into "data/textures". The implementation does not yet have an impact on drive-ability.
Thanks. This is integrated and further improved already.