200cc four stroke single engined karts have a top speed of around 60-70 mph. 200cc two strokes or twin-engined four strokes will be considerably faster so it depends what sort of kart you mean.
250cc Superkarts have been clocked at 156 mph by the Daytona City Police using a calibrated radar gun. This was at the Speedway during a World Karting Association Speedweek.Not official of course....