The biggest you can go with the stock set up is a 160. The stock is a 150. If you changed from the belt to a chain drive that will allow a little bit bigger/more clearance, but I'm not sure what the max tire the stock wheel will hold is. It is a 3" wide rim. You could do some research with that info.
Without modifications, you cannot go to a 160 if you stay with Dunlop 402. You can fit a 160 Avon Venom or Metzler 880. They are pricey though. A good price last time I checked 2 months ago (internet order) for a Metzler 160/80-16 is about $173.00 delivered.
If you want to run a 180 you can do a simple chain conversion but the rim isn't wide enough. You don't need to buy a whole wheel...just the hoop. Not very expensive at all.