Buy a new Power Supply. A fan will cost nearly as much and no guarantee it will be the correct one. Also dangerous to open up power supplies as high voltages remain even if it is switched off and disconnected from the maims. You should be able to pick up a 500W supply for around £20-25 and if your original is only 300W then it should be suitable to supply the required current
This thread is now locked and can not be replied to.