I'm outside of the dev team, but had success in porting some 3d art from scratch into the engine. Some might even get endorsed by the MWLL dev team.
I recommend Blender3D for art, although it is not really loved among some professional 3D artists.
Blender is free, all art produced with it belongs to the author, even on court. I think this matters the most.
Second we found some good tools and programs that help the way.
Here some links
wiki mapping section
one of the tools
assuming you familiar with blener3D already.
There are others, who worked with other tools that might chime in too
Consider to come to our discord channel, there one can get help with a fast turn around.