fusion isn't a particularly good tool for dealing with meshes. converting a mesh directly to a brep is particularly problematic. sometimes appears to work, only to have things not work down the road (this is a good example).
if this was ever a quad mesh there is a workflow that may work. a quad mesh can be converted to a t-spline, which can then be converted to brep. (you have to make sure you have triangulate meshes on import de-selected in pereferances to do this). this is a pretty heavy mesh for fusion though, so hard to say.
probably have better luck over all using a program intended for working with meshes. blender and mesh mixer are my tools of choice for that.
if you do have this in a quad format yo can post that and see what we come up with. (probably need to zip it first so the forum will allow attachment)
