With numbers of Ninja 250s being destroyed to do these projects, I'm sure you can get cut-off rear-end to weld back on your bike. Place an ad here, in other forums (racers), and on-line everywhere. I bet someone's got that piece lurking in back of garage somewhere.
EDIT: found one! Here,
buy this frame. I'm sure he'll take $50-70 for frame and $10 for fender. I'll run up there to get it. Chop off rear-end and send it to you! Someone else can have forks/triples, rear-swingarm and brakes for pre-gen 17" wheel conversion!