With all respect, you're one old nitpicker...
Sure nothing says it's prohibited to "mod" the game. If you can manage to implement new car models replacing the old ones and use them at home, noone will know, noone will sue you.
If you make a movie out of it and post it on youtube though, things might change. Look at what stupid kiddies do all day with speedhacks. If ONE irresponsible person publishes how car models are made and implemented, more people will start doing it, split the whole scene into modders and non-modders, build shoeboxes with infinite grip and power, ruin their lfs folder maybe, won't be able to play online anymore because of invalid parameters, will start crying around here and keep the devs off from work,..........
It may not be prohibited by law, but modding cars is not wanted by most people in the community, period. Anyone with some common sense knows that we may not modify anything, and if you ask me (I know you don't :-)) The day LFS gets moddable by everyone will be the beginning of the end for a great game I really really enjoyed for more than 4 years now.
greetz
der butz