125cc two-stroke or 250cc four-stroke are the minimums for 75 mph riding with a reasonable amount of passing power still left. The 250cc Kawasaki Ninja has a top speed of about 95 mph, with no passenger and a rider that weighs 200 pounds or less.
With a passenger, the safest minimum for 75 mph would be a 400cc Suzuki Supermoto, although the narrow seat would be rough after about 35 miles, or the 500cc Suzuki or Kawasaki.
In China, a lot of people ride across the country on 50cc scooters, but there is a lot of other slow traffic besides them. A few do the same here on the more rural roads, with some added risk from tailgaters and road rage.