DAD Kawasaki makes a 2015 ninja ZX 636. Also all you need to due to a 600 is bore it and have it re plated no sleeving is needed. cost is around $500.00 I build them all the time for bikes and mini sprints. I own Watson Racing inc. in greenwood I am the guy that builds Dustin wheeldon and Kevin Blues engines. Like I said in my last post making a bore and stroke rule will force you to use a Kawasaki 636 36cc on a small engine makes a big difference. A 600 will not compete with a 636 if tuned correctly.