Can I convert my old Digital MIDI Keybaord Casio CTK 3000 into a MIDI device, which gives only midi output
I'm unclear on whether you want to make your CTK-3000 into a MIDI controller, where the keyboard would generate MIDI signals to control other instruments, or a MIDI module, where the device would generate sound based on incoming MIDI messages. Either way, this seems like it would be a long project.
I could not find any technical data on the CTK-3000 keyboard, but I did find a service manual for the related CTK-2000 keyboard at https://manualzz.com/doc/1881381/casio-ctk2000-service-manual . The models share the same user manual, so they are likely very similar, but you'll want to look at the layouts and schematics carefully to see where your keyboard differs.
Looking at those schematics, I can tell you that converting to USB power could be hard to do.  Part of the 9VDC power from either the wall wart or the batteries is converted to 5V to run various ICs -- and some of that is likewise converted down to 3.3V -- but it looks, from my quick look, like there are some parts run off the 9V natively.  I don't know if those parts are the ones you want to keep using.
Good luck to you.
EDIT: After looking over the CTK-2000/3000 user manual and the CTK-2000 service manual that I linked to, I think I found the differences in the two: on the schematic, where there are portions labeled &quot;Not used&quot;, those are parts that are in the 3000 and not the 2000. So these should be complete schematics to your keyboard.