If your doing a over bore or a big bore you will 100% have to remove the cylinder and send it out to be bored and honed. If you were looking for more power out of a perfect running engine you would want to go stock bore high compression. The big issue you will have is the 250ex isnt a very tuneable engine and will be extremely limited in aftermarket companys making such parts because it is what honda considers a beginners quad. The engine and drivetrain are actually from a utility quad even. My .02. Welcome to the forums!
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