http://itzhakts.wordpress.com/2014/02/05/torus-earth/ gives a good treatment of the subject at hand, with a surface gravity map provided as well.
Regarding the magnetic fields, if the (presumably metallic/liquid) core was in motion or somehow had a ring current induced within, it would produce a magnetic field which will direct particles to pass through the center of the torus. The image shows the ring current of the Earth's magnetosphere, which result in the aurorae. A similar magnetic field in a torus planet would not result in any aurorae, since they pass through the center. http://media-1.web.britannica.com/eb-media/78/478-004-A8BC3FA6.jpg
But most certainly, the planet will NOT hold up under gravity without the benefit of exotic materials with tensile and compression strengths multiple orders of magnitude higher than anything known to engineers.