How do you match a VSD with an existing motor? 

It's best to start with the motor and ensure it is suited to its task, i.e. that it's not overloaded or oversized.
The first step is to define the operating profile of the load. This involves measuring the required torque and current consumption under all operating conditions.
Then, this profile needs to be examined to ensure, first, that the motor is correctly sized; secondly, that the installation is suited to the application of a VSD. The VSD must be sized to the motor load, based on the maximum current requirements under peak torque demands. A VSD cannot be sized using the kilowatt rating of the motor alone.
Suppliers of VSDs included in the ACA specified products list should help you to choose the correct product.