What happens if I use the wrong power supply?

Correct.

What will happen if you try to use it will depend on both the exact internals of the power supply and the keyboard.

First of all, a charger base power supply with a nominal output of 9 Volts/200 mA may be delivering anything - and I wouldn't be surprised if it put out >12V at no load.

If the keyboard could handle this (50/50 chance), there's still the question of how much the keyboard will actually draw. If it has built in speakers, it's likely that it will draw the 8.5 W only at maximum volume, so it might just work as long as you keep the volume turned down. However, if the draw of the keyboard exceeds the capabilities of the supply, you will likely get very distorted sound and possibly have the keyboard behave irregularly.

Summary: if you don't want to risk the keyboard, get the proper power supply. Otherwise, just try it.

You would also want to make sure the polarity is correct.

The keyboard wouldn't be harmed,

but the power supply would be overloaded and could burn out.

There wouldn't be any smoke or other visible evidence;

it would just stop working.

Get a supply rated for at least 1 A (1000 mA).

it won't work. even if the charger had sufficient power to deliver, a battery charger has no ripple filtering, so the casitone will just sound a loud buzz. But what is more likely to happen is for the charger to fail, either quietly or catastrophically. one mode of failure from over-current is for the rectifier diode to short. if that happens, then the adapter will send straight AC current to the Casio which could cause it to fail in turn.