Can I increase torque and Rpm in a DC electric motor safely using outside magnet
If you place the external magnet(s) on in a way that reinforces the motor's own magnets, you will boost the magnetic field which will boost the peak torque output of the motor.

If you place the external magnet on in a way that cancels out the motor's own magnets, you will weaken the field.

In DC motors, stronger magnetic fields will tend to generate more torque, all other things equal.  For low-speed applications, you can get more torque.  For a go kart, this is probably where you would be operating.

In truth, you are adding to the field, but significantly influencing the motor characteristics using an external magnet would take a very strong magnet since you are so far from the rotor.  Motors are designed with the rotor very close to the magnet b/c the magnetic force weakens as a function of distance squared.  You would be better off simply increasing the voltage to get more performance out.  Mind the motor temperature.