Oh one more thing. If that Ninja has stock gearing, going up one more tooth on the front sprocket will make everything a lot better. The older Ninja 250 was one of those one in a million bikes that really was undergeared. Going up to a 15 tooth front sprocket will help on both the top end and the bottom end. It's better everywhere, no sacrifices anywhere.
2002 Vulcan 750, 2001 XT225
2008 Vino 125, 2009 Genuine Stella
2012 Zuma 125, 1980 Puch Newport moped