Old midi keyboard

This:

It has only those three in/outputs, one of which is midi out. No barrel jack for power. No pots or pitchbend wheels.
If I only want simple key data, I just have to find where to put the power in. But this thing could be turned into a keytar or something similar, in which case I guess I add an Arduino and hack into the keyboard electronics.

I think it has input for PC's joystick port, and passtrough for joystick. It most likely takes
power from that port and uses it for the midi controller in your keyboard.

You should look for joystick-port specifications, I believe that there's your power.

Cheers,
Kari