It sounds like the bearings may be oiled bronze bushings, and not ball-bearings at all. When these go, they often score the shaft, and replacing the motor is the only fix. They can work for a long time if the load isn't great, the fan is balanced, and they are oiled periodically (the bronze is porous and can hold some oil). once dead, it's probably replace the motor.
You may be able to replace the motor with a different hp - size is probably your biggest issue; the output shaft diameter is common, but the biggest thing is diameter of the whole thing and length. Somebody that works with these things more may be able to offer a substitute, or maybe a call to GE.