Why would you think the police can't tell? They have learned how to tell the difference between a 48cc engine and a 66cc engine. And they can definitely tell if you are speeding, which, as far as I can see, is the only reason for such an engine. When you get caught with it, you become one more reason for the government of your state to outlaw motorized bikes. Also, no bicycle frame was designed to support the weight of such an engine. Get a Honda Trail 90/110 and ride legally.
Get a sticker that says 50cc, it's sold on the various mb sites, not that I condone tricking the poor police into letting a larger "illegal" engine slide.