For a Go Kart I think you will need something more substantial than a bicycle disc brake. If you used a go kart specific brake system like those made and sold by 'Air Heart' you can fit a single disc onto the rear axle rather than have the added weight of hub mounted brakes.
Air Heart produced both mechanical cable operated callipers and hydraulic master/slave systems.
Here is a photo of the rear brake on my Classic Racer from 1966 – a genuine fitment at the time. Quite capable to haul the bike down from 'flat out' to 10mph on the IoM!

Disc mounted on Gold Star 'Crinkle' hub
Disc carrier and hub

The Air Heart calliper with cable operation.