Replace the whole scooter. You will have problems with the motors if they are not in sync. think of a car if the wheels on the right had more power… its only good if you plan to only make right hand turns. Thus a scooter making turns with in line wheels and two speed motors can end very badly.
If you want to keep the scooter find an original motor or replace both. But replacing both is $700-1000 plus dont mess up install.. better off selling the front motor and lithium battery and buying a new scooter