Goal would be a Quaife inline gearbox. But man, nearly £1000 for one and no idea what the exchange rate is like from USD would be.
If I had to recommend someone getting a CVT gearbox for a go kart that is over built and kind of budget friendly, the Polaris 250 Trail Boss with hi, low and reverse and not what I got which was one for the 350L 4x4 trail boss. Just need the driven clutch and you're set. The Comet 94c Duster belt width should work with it. So you could have a decent setup for under $500. Drivens are like $60-80. Gearbox is like $125-200. And the Duster is maybe $200...just looked at the prices and holy crap they have shot to the moon since I last checked a couple years ago.