Instead of changing the whole rotor disc, you could also try to just change the graphical texture of the disc to something more visible.
Check in the mainrotor.mtl what transparency and what texture image (jpg or png) is used. There is a texture for top side and one for bottom side of each rotor. Maybe just decreasing transparency will do already. If not, you can play around with the texture pics using a paint program like photoshop.
Making blades/rotor disk more visible
Re: Making blades/rotor disk more visible
Modelling helis for Heli-X! See model build log here.