One very important thing to pay attention to is the amps that the keyboard draws! You need to find a power supply with the above discussed specs but with at minimum the number of amperage the manufacturer gives. Using less will put stress in the power supply and the keyboard and probably this will result in sever damage in either or both. Personally when I do things like this I aim to use a power supply that its amperage is rated about 20% more than what I need. But this is because the power supply will run a bit less stressed. So make sure the voltage is exactly the same (not less not more) and the amperage is either same or greater.
One detail that is also very important is if the keyboard takes DC (direct current) or AC (alternative current). Probably it is DC but older devices (like the atari 400, 800 and their disk drives 810, 1050 etc) used to take AC and were converting it internally to DC. If your device needs AC and you give it DC, or the other way around at best case it won't work, worse case you will damage it (and/or the power supply).