Motor Running Than Rating Speed

The answer may be specific to the motor manufacturer; it would be best to ask them.

If the motor is capable of operation at the higher frequency (and presumably proportional voltage of 828), it should be able to produce 300kw. However, the power consumed by a fan varies as the cube of its speed, so the fan will need (1.2)³ = 1.728 times as much power at the higher speed.

We don't know if the fan was selected with these considerations in mind.

If you don't pay heed to Tornado's post #4, you are likely doomed to failure. The motor speed itself is, as he says, a question for the motor mfr because it has to do with the bearings they used and the motor cooling system. Nobody else can answer that question for you.

But your line of questioning is like asking about the dinner menu on the Titanic when you should be asking about the number of lifeboats. When you over speed a centrifugal fan or blower, the power REQUIREMENT from the load increases at the cube of the speed increase. But a motor power output is fixed by the available V/Hz ratio. So if you have increased the Hz but have not increased the V, because you got to maximum V at the design speed of 50Hz, then your motor kW has not changed and you have LOST torque, just at the time when you need it most. If your system was designed with this in mind, then it may be OK, but there are a lot of ignorant people out there who think they can just turn up the speed on a fan and get more flow, they end up with failed applications or damaged equipment. Start asking questions of your vendor there.