Nice photoshop Craig! Don't know about Gimp, but Photoshop has this 'healing brush tool' in which you can sample part of your image with Alt, and copy the pixels-color difference by clicking... You might be able to use that for the boot lock and wiper. For the wheels there's a color replacement tool, but I don't think it works well if the old color is grey(or black/white) since it only works on colors to other colors... Might be able to use quick selection tool to select the entire wheel, and then (with anti aliasing enabled) use the brush to color them?...
@Jamie: No idea mate on the engine, give it a good service(new spark plugs etc) and it'll at least last longer and since that'll make the car happy, it'll also make it 1mph faster jk...
But as far as I know the wings are either bolt + spot weld or only bolts, since I know at least the top has bolts in it...
Almost forgot:
Pic whoring time...
(Somehow, even though it's just ice and snow, I think the matte white/red roof looks kinda funny
And then my attempt at doing an art....
I think this one is best though...
BTW that's my house in the background of the last pic...