None really exist. Outside of using a divorced gearbox with high/low, reverse, there isn't anything that exists as far as several gears. You would be better off getting a single cylinder engine from say an ATV, dirt bike or motorcycle and using that.
I've only ever seen one gearbox that was made for a go kart, but as far as I know, it hasn't been used on a kart. A long time member on here, who hasn't been around for a good while now, Tmans Karts, got a motorcycle engine that was stripped down to just the transmission by a fan of his to use on a kart. This was 2 or 3 or so years ago. He barely posts anything on his YouTube channel anymore (has kind of faded into obscurity). Otherwise, Rather B Welding has been working on a 3 speed with reverse, but nothing much in a bit since then.