What pulley size on compressor?

I currently have a 300 litre compressor with a 3 phase motor. It is 7.5hp and 2860 rpm. I only have a single phase supply so want to fit a single phase motor rated at 5.5hp. I was thinking of putting a smaller pulley on the motor to compensate for the lack of power. The large pulley on the compressor pump is 430mm the small one on the motor is 185mm. Thanks if you can help.