I have uploaded my latest Alpha version of a displacement map editor. The goal is to be able take random terrain that you create and lay a track path down on it that follows the terrain like a bulldozer and plows a path through the terrain. It's adjustable as to how much it follows the terrain and how it handles the edges. Future plans are to provide touchup tools to provide ways to dig out and build up jumps. Ultimately I would like it to output a texture that shows where the track is.
I see this as an ongoing project and it isn't tied to a specific version of Motocross Madness. Right now it is working with 257x257 displacment maps and 960x960 textures. It will take larger ones going forward. I have it defaulting to my MCM 2 track Valley Ride. If you want to start out with a blank slate the best thing is to just paint up a gray scale dispmap with bumps and random terrain and fly around on it. You can use the default grass to start out with.
I would love to have a few people take it for a spin and give feedback. I will be posting a thread in the forum to explain a few things .

Before applying path:
After applying path:

Displacement Map Terrain Editor Help file
Displacment Map Terrain Editor