Some ninja 500 / 250 swap threads:
The Ex 500 Engine swap with pics
Custom 09 ninja 250 with ex 500 or 650r engine + others
Unless someone has a free 250 and free 500 already in their garage, and unlimited free time, I don't see the point of the exercise.
A new Honda 500 is $5K, as is a Ninja 300, both of which would outperform the frankenbike.