Your gear ratio will depend on a lot of things, I cant make a guess without a lot more info.
Is your stock engines governor still hooked up?
(if not, do you have a tachometer? How many RPM does your engine to rev to?)
Whats the name of the oval track you'll be racing at? (and do you know what class you'll be entering?)
How tall are your back tires?
(air them up to 12lbs, then wrap a tape measure around the center of the tread to accurately measure their circumference. measure both your back tires like this and post your findings)
Also, do you know if you wanna run #35 or #219 pitch chain and sprockets?
Do you have a race kart? (post some pics/info)
Also, how how much will the driver weigh?
What gears do you have now, and how did they seem to do racing/practicing on the track, against the other karts in your class?