Well, the right thing to do would be to clear it with the LFS devs first.
If you had a 'project' where you'd use some LFS content in another
game or environment than LFS itself, if it's promoting LFS in a way,
you could be allowed to do many things.
As mentionned, the SMX format is a viewer format only. It has little
relation with the actual game format used in LFS. That didn't stop me
from porting Blackwood to Netkar. I had to make all the background terrain
myself and used actual textures from LFS to retexture the whole thing, but
it worked well and the track is fun to drive in another game imo
I felt a cold when asking for other tracks though, so i stopped there. I
would have loved to do all the tracks, since at the time, S2 wasn't out so
Netkar was also a way to drive a F3000 and 360 Modena on LFS tracks