Hello
. So from what I see there is two issues
1) you can't use blender material. They aren't compatible with SuperTuxKart materials. You must use what's described in the track maker guide. That's what makes the problem of invisible leaves
2) You should change the normals of the terrain
. SuperTuxKart will optimize by only rendering one side (the side where normals points). You can use CTRL+SHIFT+N and CTRL+N to change the side of the normals.