I'm not sure if this is your exact model of bike, check it out yourself but I had a look at the parts manual in the link and it seems that on a ZZR600 you can replace just the bolt that you snapped by itself which is handy. Take the axle out, take the adjuster out, take the broken bolt out, put it back together. Should be easy. https://www.oemmotorparts.com/oem5.asp?M=Kawasaki&T=ZZR%20600&Y=2004&L=KA-ZZR600-8&O=Swing%20arm&F=KA-ZZR600-8_BF&L2=KA-ZZR600-8_BF You could still ride it the way it is if you really have too but you won't be able to take the slack out of the chain. It could be too slack to ride safely. If it's very slack you would risk it jumping off the sprocket. It's up to you if you think it's safe or not.