A lot of it depends on how heavy your kart is. Id gear it 5:1 and rev it to 4000RPM. But thats just me lol
With that kind of power Id go 5:1. With that you'll have good acceleration and top speed. And if you dont like the acceleration you get you can always play around with what you like between 5:1 and 6:1. Im going to gear mine 6:1 and I only have a 9hp motor. It also depends on where you ride. I go through a lot of deep sand and I prefer really quick acceleration over top end. But I think youll be really satisfied with 5:1 with an engine like that. Good luck! I hope ya post up some vids of it, Im interested to see how it turns out!