I went with Doc Yo's suggestion and used some more Kotobukiya detail parts to build up the rotor heads. They're much sturdier now and look believable, I think.
Rotor mast before primer:

The blades are held in place with a straightened paper clip, cut to size. I probably won't need to glue them in.
Here's a detail, after a primer coat:

The two thin rods go all the way up to the top rotor. While not clearly visible, I wrapped some 30 gauge wire wrap wire around the now thicker plastic rod that acts as the axle. It looks like a spring and acts like one.
All together:

Now I have to clean up all the scratches and gouges on the blades and start on the cockpit.
Thanks for all the comments!
Frank