No -- Yamaha distributes Cubase AI just with their synthesizer line -- I got a copy that way when I bought an MX41.
You should still be able to use your Cubase AI software (assuming you still have the license) with your SX900, although I suspect you'll have to upgrade to Cubase Elements to get mapping support for the SX900 voices (unless you're willing to spend the time to do the mapping yourself ;-) ).
I'm debating myself if I want to upgrade Cubase for my SX700 -- I'm still trying to determine if Cubase has the necessary maps for the SX series....
[EDIT]
I just went into Cubase AI to look how things are setup to explain myself better:
If you go through the menus: "Devices >> MIDI Device Manager"
You'll see that you have a "Installed Device" for your Motif (for me it says "Yamaha MX"). These installed devices are configured with a setup XML file (you can see buttons for importing and exporting these). The setup files provide the necessary mapping between voices and commands between Cubase and the instrument.
Click on the "Install Device" button to see a list of the currently supported instruments for your version of Cubase. If you're lucky you might find "Yamaha PSR SX" in the list -- for my old circa 2017 version, it's not present.
If it's not in the list, it *MIGHT* be included as part of one of Cubase's more premium options like Elements or Pro -- I'm still trying to determine that myself...
