I bought this nice synth to a friend in need for a few bucks. It is really cool, it is kind of retro but I decided to add more retro and a pot to change tune easily. I attached some pictures.
So I'll so you how to hack it easily. This will probably work with all synths and keyboards with a tune preset.

Step 1: Materials

The things you will need:

X1 50K potentiometer (check your preset pot to confirm value)
X1 Knob (optional)

Household items:

Drill
Wire
Retro style wood
Screwdriver
Dremel

Step 2: Making Space for the Pot

So go ahead and unscrew the synth. I got 6 screws, remember there are always hidden ones so check well for screws.
Then locate and drill a space where the pot will fit comfortable.
With a dremel clean the space around the pot's place so it can fit secure. In my case it was just in a spot where the screw was so crushed the screw place away.
After cleaning procedures make sure your pot fit well enough it can move freely.

Step 3: The Pot Issue

So once you have you pot inside we have to locate the actual pot for tune. If you synth can be circuit bend in any other way, don't be afraid to try it.
I soldered 3 wires to each terminal, about 15 cm each wire. It was pretty easy to locate the other pot.
I went ahead and destroyed the pot with some tweezers. Remember if you do not destroy the pot you will increase the value:
50 + 50 = 100
Yo have to make sure to eliminate 50K because that is the value needed.
50 = 50

Solder to the original pot terminals, the wires, matching the polarity of the built in pot.

Step 4: Closing It Up

Tape the wires together to make sure you don't have problems later. Close everything up, put screws back on.

Step 5: Finishing Retro Touches

To add a retro sense I found a piece of wood I didn't need so I used it to add retro felling and drilled some holes so you can still listen to the integrated speaker.

Step 6: Wrap Up

So it's really easy to add a little something to a synth. The best thing you can decide the shape and size. You can also add a knob for the pot.
Write in the comments what you did:=)