You can install a 15t front sprocket, and start out with a 30t rear sprocket on the Rebel.
I presume that you have an older model. Mine is a 2013 model, and the engine works a lot better after doing some basic tuning.
I regularly hit 80+mph on mine (rebel), and with a long road, and a little wind in the back, I can go 90mph on it, topped out at 92mph on the interstate, and that is with trunk and luggage cases.
I'm also looking at a bigger bike to upgrade from my rebel, because I can't always count on 85mph, like when the wind is bad it does only 80mph.
Here are a few things you can do to the rebel to make it faster:
1- The Rebel makes its peak hp around 7.5k rpm and it's peak torque from 6.3 to 6.9k rpm.
You must use at least a 15/30t setup, or a 15/28t sprocket setup, to get closer to these rpm ranges at top speed.
(it comes stock with a 14/33t setup).
With a 15/30t sprocket setup, first gear actually becomes useable, and you can still flick the clutch into first gear from a dead stop.
With a 15/28 you'd have to be more gentle.
In the case of a Rebel, lower RPM (@6.6-7.5k rpm instead of 9k) means better top speed performance.
There's no benefit running at 9k rpm top speed, when 12.5% of it's hp, and 35+% of it's engine torque is eaten by friction at these higher rpms, and re-gearing will make it feel like a whole new bike for an only $70 investment, not to mention with a 15/28t, a lot of the vibrations at speeds below 40mph will be gone.
2- After the sprocket change, You can lean with your left elbow on the tank while riding, duck forward, and put your feet on the passenger pegs instead of rider pegs. This will add at least 5 to 10mph on your top speed.
It makes little to no difference on the stock gears, because your bike is rev-limited, not limited by aerodynamic drag at wide open throttle.
3- Valve adjustments need to be done every 7.5k miles
4- keep the bike as much stock as possible. Aftermarket exhausts can degrade performance, if you don't rejet the carburetor.
5- Add a little 5w30 in your oil (stock oil is 10w40). I live in South Florida, and I dose 75% 10w40, and 25% 5w30 in my bike; roughly estimated.
The main speed improvements will come from putting your feet on the passenger pegs, and re-gearing the bike to run at lower rpms.
There's no substantial performance benefit gearing higher than a 15/28t (or 15/27t on the newer models) on this bike.
If riding comfort, and mpg matter to you, your next step up would be a Honda CB300F.
It won't be a big step up performance wise, since it's a single cylinder engine, but on average it does have about 5-10% better performance from a rebel.
The R3 is more of a noticeable upgrade, but even though the bike has more upright handlebars, I find the R3 still has a too much forward lean for the rider.
It's a lean more suited to halfway meet between track riders and road riders.