As far as installing Cubase, you might be able to download a free trial from Steinberg's web site and then activate it using a license key.
In fact, whenever I've bought audio or MIDI equipment that was bundled with a free DAW, the DAW never came on a CD with the equipment, but always had to be downloaded from the web and activated with a license key that was printed on a card that came with the equipment.
However, if it's a used MOX8, chances are that the previous owner already registered the Cubase license that came with it, in which case you won't be able to register the same license.
Besides, the license was probably for an older version of Cubase, anyway-- although I think Steinberg's web site has a page with download links for older versions of Cubase.
I'm currently awaiting delivery of a Yamaha MX49, which is supposed to come with a license for Cubase LE. It was a restocked item, which means the person who originally bought it and then returned it might very well have registered the free Cubase license already. In fact, I suspect that some people might order a keyboard and then return it just so they can register the free license before returning the keyboard, as a dishonest way of getting a free DAW.
Anyway, I already have Cubase Elements, so if by chance my "new" MX49 actually does include an unused license key for Cubase LE, I'd be happy to give it to you-- although you might not be able to tell whether the license key has already been used until you try to activate Cubase with it. I'll post an update after my MX49 is delivered-- and if I do get a Cubase LE license with it, I'll send you a PM.