If it is just for the fat tyre, you can put a 170/60 one, it is actually the perfect size for 5" wheels. Also there is a member who put a Thunderace wheel with spacers (the axle is thicker).
I suspect that it is possible to change the bearings of the thunderace wheel to compensate, no spacers needed. Even better, I think the FZR 1000 Exup wheel is straight fit, but not sure.
TRX850 front wheel (120) is a straight fit too. I am very happy with the 120/70 - 170/60 set. I did not like the 120/70 - 160/60, the bike felt worse than standard, at very fast corners it wasn't good. So it is possible that just changing the rear tyre feels weird too. The member who first put the TRX wheel was happy with a 120/60 - 160/60.