Damage is set in the txt file, not the gmf. That explains why you can't change it.
If you want to change the shape, the Astroflight is probably one of the hardest motors you could have picked. Best way to work with it is to copy/paste all of the MESH_VERTEX into Excel and apply a transformation formula (for example, multiply everything by 2 to make it twice as big). Trying to change the shape by editing each vertex by hand is far more work than it's worth.