Cracked engine cases are never good. Because the engine on the Ninja 250 provides an integral part of the frame stiffness, I would not recommend riding it with oil leaking and a crack in the engine case. Cracks do not get better with heat, they get worse. I would not use JB weld on it at all. Engine replacement or getting another bike are your options at this point. Get another bike and you'll have plenty of spare parts that you can use or eventually sell. Selling used Ninja 250 parts is a big money maker. A 1/2 dozen parts can net $1k easy.
A replacement 250 engine is getting hard to find nowadays.
Last edited by DaBlue1; 05-10-2013 at 06:01 AM.