I wouldn’t hesitate for a second to replace that cheapo 2 conductor 16ga cable with a good, doesn’t have to be expensive, 3 wire after market power cord. I would probably look at, minimum, a 12ga conductor power cord. Try before you buy if possible... You will hear a difference for the better!
As for the ground wire in the cord. The IEC inlet connector on the back of the Marantz receiver does not have a ground blade. Two blade only.