Not tried but you could use the Separate tool ( Mesh > Separate ) in order to, well, separate the parts of the meshes, since basically the proxy geometry from the shooter game is done by joining different geometry which actually is already separated, but Maya consider it as a single mesh.
If you use that tool you’ll end up with different parts, so then you can simply removed the upper part of the body…
Not tested in Maya but it should work…
PS: If you’re not able to select the meshe itself check if its on a separate layer or if its in reference mode 