if you want to build a bike, the ninja 250 frame is probably the worst you could pick. steal is heavy, hard to machine, and rusts like hell. its an engine supported top diamond frame which means its not stiff at all and flops all around and will bend from setting down wheelies or jumping it. not to mention its a one-piece instead of having a detachable seat frame.
if you want to build a bike, start with a good foundation. get a b12 frame or something thats easy to work with
heres a good example of what can be done with a b12:
