I want to put a bigger engine in my Ninja

This question comes up quite a bit. If you'd like to try, go ahead. It's not generally perceived to be a good idea. If you have success, please report back to the board with pictures, and be prepared to answer some very pointed questions.

Look at the frames of the bikes from which you'd want the donor engine to come from. Notice that they have a Twin Spar frame? Now glance at the frame of the 250... See that it's different? A swap isn't going to happen. Besides that, what do you think an engine with triple, or more, the power would do to the stock 250 frame? It would crumple the thing like tissue paper, and all your work shoehorning a Yamahonsuzaki 600cc SuperScreamer engine into the frame will be destroyed (if you're lucky) or you'll be killed (if less lucky).

Along the same lines, the collective admins have been around here for 6+ years on average. In that time, every few months or so someone comes along and wants to supercharge/turbo/nuclear power their EX250. They're sure they can do it and go 800 mph. No problem, because they're smart.

Well, Mike Norman at G Force (no longer in business) built race motors adhering to production AFM rules, meaning stock exhaust, airbox and carbs (with jetting) that developed enough horsepower to easily overwhelm the rather fragile bottom end. A large number of cranks and bearings were destroyed, and while he eventually (nearly) perfected the process, they were still small bombs waiting to go off. There is an inherent limit to what you can pump through a Ninja 250, and from hundreds if not thousands of dyno runs, we know that number is in the high 30's of rear-wheel HP.

A supercharged/turbo'ed/nuclear powered EX would likely exceed that limit easily, and for that very reason the admins poo-poo those who suggest it. It should also be noted that such (non-super/turbo/nuclear-charged) improvements from G Force cost thousands of dollars -- it wasn't a simple matter of bolting on a few parts or overboring the engine.

If you can't be happy with the 250 in Stock form, you're not going to be happy with the bike at all. Sell it and buy what you Really want. It'll be MUCH cheaper, more reliable and less stressful.