But one thing puzzles me about the original painting: where are
the radiators?

I THINK this picture captures the radiators on a torus. However, I'm
currently blocked from zooming in on the picture.
http://www.dyarstraights.com/msgundam/STANFORD.GIF

The fins at the end of the long tunnel coming off the hub (Fig 1a) are
the radiators:
http://spaceset.org/p.train6.html
http://www.olympus.u-net.com/torus.jpg

Note the 4-way "solar panels" of the Bernal Sphere (Fig 2) are also
radiators.

Would a habitat of this size need to have substantial
radiators to get rid of excess heat?

Absolutely.

Mike Miller, Materials Engineer