You should be able to hit 30 with that combination. When I do the math, you are basing your top end on 3600 RPM. That is the governed speed of engine. It is probably capable of (at least) 5000 RPM which would give you 30 mph.
A few things I would do.
1) Make no mods at this time.
2) Assemble kart to get it rolling and everything operating as it was designed to operate. Meaning: brakes-throttle-steering.
3) Test run kart
4) Once you are sure everything works as it should, ride it for a couple hours using the goverened speed to break in engine under normal conditions. Any other problems may and probably will show up. Deal with them one at a time until you think it's perfect.
5) When everything is 100%, you can start to think about modifications to increase speed, such as bypassing or eliminating governor, bigger carb, sprocket ratios, clutch changes, tire sizes, high performance engines, exhaust...on and on.
Just remember that it has to run well in its "stock" configuration first, then comes the bells and whistles. Took me 9 months to get my kart where I want it. Now I will start to make the major mods such as larger engine, higher compression ratios...etc. etc.
Let us know your progress.