i dont quite understand what your broken engine has to do with the big bore kit,
did they explain what was wrong with the engine after they fixed it?
as for the hard riding, if im not mistaken, ive read reports of others driving their lil ninja hard all the time, without any issues. occasionally i push my bike hard too for extended times when i got for a sprinted run around the island, and its always done it flawlessly. but i dont drive that way all the time, since it would just annoy a lot of people and the roads are not good enough nor empty enough to drive that way, hehe