gR33ddy as far as what you can and cant have, its completely up to you. Technically anything different from the stock specifications is classed as a defect. All mods should be ideally engineered to pass road regulations (But u wouldn't waste your time). Most police will look for the most obvious defectable parts on the engine ie. large turbo, fmic, chrome piping and BOV's. If you modify a vehicle try and do it with boltons that dont stick out and u will probably get away with it. Then again if you are really concerned about it, spend a fortune and get mods engineered and keep papers in the glovebox. This way you dont have a problem.
I have many obvious engine and vehicle modifications on my vehicle but unlike the rest of you, i dont get the pleasure of being hassled.