I found a Marlboro -font- off a website ( [
www.sharkshock.com] ) and with that you can make your own Marlboro logo's quite easily. That's just an example, you can download various famous fonts like that off many websites.
I've personally made the Ferrari a fantasy Ferrari - just changed the colours and re-arranged/added the Malboro logos but I'm also in the process of making the Prost team into a complete fantasy team. Because the TGA files look so weird, I'd recomend that you only make alterations in places that you
know correspond to various part of the car. Like usually you can make out where the the nose and sides of the car are. Best way is trial and error though - although to constantly completely change the .wad file is a bloody pain...
Hope this helps, although I'm sure people have got more of an idea than me