[Eliot Ness]

I'm just going to toss out a couple of things and maybe one will work or it might give you another idea.

I'm not sure how well this would work, but you could try something like Milliput or Plumber's Epoxy and make a new one.  This stuff is a two part putty that when mixed makes a substance that can be molded and sculpted like clay.  You might only have 6-8 minutes before it starts to set.  There are YouTube videos on Milliput you can watch and get a feel for how it works. 

The other idea is to find some plastic, epoxy, or PVC tubing (hobby shop, eBay, or plumbing section) with the right inside diameter and mold some of the above putty  inside and shove it on the flywheel.  Then cut some shallow slits on the surface for the belt to grip.  Or mold some putty on the outside like the surface of the broken piece.

And a bonus third idea (and probably the best).  Place a classified ad for one on here and the other radio forum.  Include both pictures and I'll bet someone has a parts chassis they can get that piece off of.  I had a ratty '42 Z console I drug around for years picking parts off for myself and others.  Veneer, dial glass, band switch, tuning knob, &  power transformer, it was a shell when I eventually have it away.

Edit: after doing a search and seeing where that piece goes I think Murf's idea of wrapping some friction tape around the shaft has merit at least as a temporary fix.