Here is the start of what i know is going to be a big task...

All i have done is put a new rear bumper on and made side skirts larger...and debadged

mmmmmmmmmmm...

the line is abit wonky...

if you've still got it in psd (layers) then you can make the bumper straight... and an easy way to get the colour right it to desaturate(de colour) both layers and mess with the brightness of the bumper til it looks the same as the car... quite simple that way...

should look good!!