With your present set-up unless you get a specialty racing clutch (expensive) you're pretty much going to end up with a 12 tooth clutch. That ratio would give you 22 mph @ 3600 rpm. If you keep the motor governed, that'd keep the kart pretty safe for kids, and you won't risk popping an expensive motor (Kohlers aren't cheap at all). Noram makes clutches with higher tooth counts if you want more top speed. (16t would get you to 30mph at 3600 rpm, 8hp would have plenty of grunt to do so.)
If you had that kind of money though- I'd go with a comet torque converter, which would climb trees with your set up. Is that 85 tooth sprocket welded on?