Because the mesh is fairly simplistic in terms of shape, I would recommend rendering out the Z-Pass/Z-Depth  (setting your camera to your top view before render), and saving that image. Then you can start wit a new plane, and use the displace modifier, using that rendered image as the displacement texture. Once you have that in place, you could subdivide your mesh until it fits your needs, and add sub-surf modifiers to smooth it out.

I will attempt to circle back around to this post shortly, but as time allows, in order to post screenshots of the process, and step by step text to finalize this answer.