Well, your in luck. I've seen this work flow go terribly wrong, because the originating mesh file is usually all screwed up. But in your case the mesh file is clean, and primarly made up of quads.
Honestly, a mesh editing program like blender is probably better suited to this, but if you want to keep it all in fusion, then the next step is to convert it to a t-spline.
Right mouse click on the mesh body in the browser and "convert quad mesh to t-spline" to get a work able t-spline.
There are a LOT of sublties in volved with working with quad meshes and t-splines, that aren't always apparent to folks new to the process, so you might consider spending some time learning t-splines.
A couple of suggestions for this model-
-I noticed when trying to convert the t-spline to a brep that there is a problem around the vents at the jaw area. That needs to be cleaned up. (the issue is there are faces/edges/control points are overlapping, so it won't convert)
-for a model like this, you need to spend most of your time in box mode to prevent adding more errors like the one pointed out above. (if it looks good in box mode, it will look good in smooth mode. the opisite is not true.)
-you might consider removing geometry like vents etc, that interupt the flow of the surface, and add them on as seperat bodies with a "combine" command. (That's going to be time consuming, and depending on what your trying to get out of this may be more trouble than it's worth. It is how I appoach new designs though.)
-try and get rid of as many triangle faces as possible (quads are prefered.)
