Had this with my old NX650 Dominator - does "Whizz, whizz, crunch!" sound about right? Easy check now you've got all apart is to put the parts together on the bench and then try to turn the big cog both ways - it should move freely one way and not at all the other - any movement (even 1mm) in the 'wrong' direction means the rollers are f***ed.
As to where it can be got, have you tried CMS over in Holland?
Have a link on meI think it's a common part throughout a lot of the Honda 500/650 singles (I'm sure more knowledgeable ones will be along soon if I'm wrong), so a hunt around using XBR and NX as the start point could be worthwhile.
You're welcome.