You will need to rejet your carbs for the non-stock pipes you are running plus any mods to the air box you have done. Simple power mod is to drill two 40mm holes in the upper lid of the airbox but rejet with plug chops if you do this or you will run lean and seize!
Fwiw, the mc-16 engine is rigidly mounted to the frame, all other nsr250's are rubber mounted. The mc-16 is also much narrower than later models so fairings and tanks need to be modified to fit. How do I know this
have a stock mc-16 in the garage along with a modded mc-21sp.