A 17/47 on most Hawks should be able to easily hit 65mph and will cruise along at 55 nicely, without the engine sitting at 8000rpm to do so.
If you go to
www.gearingcommander.com and select the RPS Hawk 250, you can play around with sprocket combinations and tire sizes to see where you end up at different RPM in different gears.
I used it to look at a Hawk on the stock rear tire size with a 17/47 sprocket setup and it puts you at 63mph right at 7000rpm. So it should be able to hit 65 right around 7300, and in theory 70mph at about 7800. with a 45 rear sprocket, 65mph happens at 7000.
In my experience, top speed, regardless of the sprockets (other than the stock), tends to happen right at power peak, which is right around 7200-7300rpm. That is why most people with the 17/45 top out right around 68mph. 68mph also seems to be the limit of the CG250's ability to force a Hawk through the air, because after that, only the smallest and lightest riders can squeeze a couple more mph out of them, even with smaller rear sprockets.