The bike as geared will theoretically do 93 at 5500 rpm. Good luck on getting it there. Putting a one tooth larger sprocket on it would raise the theoretical top speed to 98. However in reality you may LOSE a bit of you 83 mph top speed doing that because you aren't as high up in the power band. the thing you are lacking with this bike, my friends, is horsepower. All the gearing changes in the world aren't going to make it goo much faster, actually if you geared down one tooth you may pick up a couple of MPH because you are working in a higher RPM/power range.
If you want to go faster on a Royal Enfield, your only choice is to hot rod the engine. Build a 535 and add cams and more carburetion, better exhaust, etc. The gearing changes ain't gonna cut it, guys.
Bare