Hi Tee,
Since the previous owner used an ac-adapter from a pedal board, we must assume that the correct voltage of 9 V DC was used. The unit should be diode protected internally against reverse polarity, but NOT against overvoltage!
Now, the Casio MG-510 is from the late 80s, so it probably needs some recapping ( new electrolytic capacitors ). Not so hard to do, but If you're not familiar with soldering etc. I suggest you to visit Grant Muller's site here. There's a very thorough documentation on how to recap an MG-510:
Casio MG-510 Midi Guitar | Grant Muller
Next you could study the service manual in detail. I've just uploaded it to this group. Have fun and good luck.
Bo