Sorry but this is the best I can do with the file at the moment and it took about 3 hours to get here. I could either give it a thin wall or a really thick wall. As for what I did,
1) I started with the .stl file you provided and put it in Edit Mode in blender,
2) deleted half the Model
3) deleted inside polygons that were extra in my opinion
4) ran it through an add on I use in Blender called "Mesh: 3D Print Toolbox" which checks all the potential print problems
5) selected all the intersect polygons
6) hit CTRL F
7) Hit Intersect: Knife
8) on the left side where you see the options for intersect, Under source, select "Self Intersect"
9) give the model a mirror to complete it
10) smooth out all the spots that need smoothing
11) send the model to Makeprintable.com - They will close up all the holes and fix a bunch of problems, but it is limited.
12) bring the model back in
13) add a "solidify" to the model to increase the internal wall thickness . In this case, that didn't work (resulted in thin wall with imperfections on the outside) so I made a copy of the model in Blender, scaled it down slightly smaller, reversed all the normals, joined the models together, bridged the gaps around the neck
I uploaded a zip with my blender file that has several models inside. One has the thick wall, one has no wall. I also included the thick wall in one piece in case you want to resize it. Also, I cut up the thick walled helmet for a print bed.
That model was tough to salvage for me. I'm still new at blender but boy, what a pain. Hope the wall thickness isn't a dealbreaker for you. Otherwise, you can use the model that has no wall thickness to get to the size you want.
