Full set of metric tools – allen wrenchs, open ends, sockets. Spare 5 & 6mm allens and 10 & 13mm open ends. Long socket extensions for removing wheel nuts.
Flat screwdrivers, including small ones for changing jets. Phillips screwdriver if any of your hose clamps need them.
Dykes for cutting zip-ties, some pliers and such (especially needle nose)
rubber mallet
tire pressure gauge
axle removal tool (or a lot of patience)
Rotax clutch and piston stop tools (I think I remember seeing a rotax engine on the picture of your new kart)
Tire changing tools, including bead breaker
Spare nuts and bolts, zip-ties, etc.
You’ll add more to your toolbox over time, of course, but that’s the basics.