Looks like you need to clean up the water pump. Note that there’s a plate behind the gasket that you are seeing with another gasket then the case. Be sure to take the plate off too, clean everything up, and use new gaskets when reassembling.
However, you have a different problem that’s not causing the motor to start… The motor should start even if you have no water in it… let alone a leak.
Spark: Check that the kart has spark. While on the kart stand, pull the plug, connect it back to the boot, hold the plug against the cylinder head nut, put the kart in gear, and turn the wheels to turn over the motor. It should spark.
Fuel: If it has spark, be sure it’s getting fuel. Put a new plug in it and see if there is fuel on the plug after a few attempts to start.
Compression: If it has fuel and spark… check the compression. If you don’t know how to do this, just take the cylinder off and see if the ring is stuck to the piston or excessive wear.