While this thread is a couple of months old already... if anyone is still thinking about this option....
The 500 ELM and 500 ELX motors were never designed to run on that much current (9volts). Even the newer 553 ELX takes only 7.5 volts (5 - AA size batteries).
Running the motor with 9 volts, ... yes it may get you a faster frame rate, but it will burn the motor out much faster.
While NiCads are not made anymore, there are other solutions. There is an adapter which takes 2 CR2 batteries which is about 6 volts (the same as the original NiCads).
Bottom line 9 volts is way too much for that motor to handle.