I'd go 14/38 keep in mind that as a rule of thumb +1 in the front = -3 in the rear and -1 = +3
The mojave 250 has the same motor as a ninja 250 they just changed the cam and carb I know the ninjas dont redline till 14k not to mention its a 2 cylinder so hes going to be able to wind it out, not till 14k but still pretty high so your probably going to have to sacrafice alot of bottom end to get him
on the topend