Hi Adam. The motor in the wastewater pump can be used with a transformer, without rewinding the motor.  The motor rated for 120V 60 Hz will work with 110V 50 Hz. You can get a transformer which will step down for the pump. 

Most induction motors operate well within a moderate voltage range, as long as the volts-per-hertz ratio is not deviated.  The motor is rated for 120V @ 60 Hz; which is 2V/Hz.  Since you have to work at 50 Hz, to keep the ratio, your transformer will need to provide 110V@50 Hz, keeping the same 2V/Hz ratio.

The other concern will be the pump impeller speed. The pump impeller is engineered to run just below synchronous speed for 60 Hz, which is going to be 1800 or 3600 RPM.  When operated on 50 Hz, that will be operating a little below the 50 Hz synchronous speeds of 1500 or 3000 RPM.  That would mean the pump would not be able to produce as much lift or head pressure due to the 20% reduction in impeller speed. 

The speed of the impeller won't be changed by rewinding the motor. That would only change the V/Hz ratio for the motor. 

Hope this makes sense! 
Sincerely,
David

Hi Adam.  Unfortunately adding motor HP won't make up for the speed loss. You'll need to increase the impeller diameter to get equal performance at a lower speed.