Hi,
My recomendation is to try to portrait the damage of the existing model with painted textures first using 'Photoshop' and then you can add definition with modeling.
Be sure to search google warehouse to see how other people are making this types of models but above all check the 'Gallery' and 'WIP' on sketchUcation because I have seen some advance stuff there. If you want constructive feedback make you own 'WIP' thread to show the progress of your work.
Cheers!