Well, ms Tanya, the seat height of the 250 Ninja is only 30.5" and it weighs 374 lb. The 650 has 31.1" seat height and weighs well over 400 lb. Ask yourself if you can handle those numbers.
You might be a bit tippy-toe! That's not a good thing to be with a motorbike! The dealer would know if a lowering kit is available for any model. A good upholsterer can certainly lower the seat height by cutting down the foam padding, maybe an inch or two...though there's not much padding to begin with on a sport bike...