Edit2: Compiled hex files - https://drive.google.com/file/d/0BxyIgc7ZORksdUhLNGdoTFJXQzg/view?usp=sharing
Using the method in this previous post you are able to replace the IC on these Simple Jet boards. The issue is the ATtiny24 only comes in an SOIC narrow variant. Looking through the ATtiny catalogue there also aren't any 18 pin SOIC variants available.
It's time to get crafty again. Looking around I found the ATTINY261A-SU which is a 20pin ATtiny variant for $1.58AU each in when you order a quantity of 2 from RS including shipping. So still quite cheap (you could use the more expensive ATtiny26 too).
The problem is that the chip itself has a layout of 20 pins.
Here are the new pin designations:
I am still using version 2.1 of the code as before. Here is the updated source code (precompiled hex here; note the EEPROM file is labelled .eep). Programming using the guide from the original post. The fusebytes remain the same:
* fusebyte low: 0x42* fusebyte high: 0xdf
Once programmed, don't power the chip on yet. It's now time to modify the PCB. You will now want to remove the components marked in red and blue on the left/bottom, and desolder the main IC. To make it easier, you can cut each leg of the IC carefully and clean up the pads once the chip is removed with some solder wick. Be careful not to lift any traces.
Now solder the newly programmed IC in the orientation shown and bridge the contacts marked in blue with solder and (pin 10 and 11 should be at the bottom and not connected to any traces as shown in the picture above). You will then need to connect a 10KOhm resistor between VCC (pin 6) and pin 10 of the ATtiny (as shown in yellow). Additionally bridge the other points marked in yellow with some hookup wire (don't connect the two wires which cross over together, they should be separate connections). Once soldered, you can add a little hot glue to pins 10 and 11 to give them a little strength if you like.
Note that there are some functional differences as outlined in the above table and the source as to how to re-calibrate and invert the X-Y axis. The re-calibration pins have changed and the invert axis pins are now enabled on active high, rather than active low (active low is the default when you follow the instructions above). If you want to invert, you just need to desolder and lift the appropriate axis pin. Please see the above table's "new code description" column for details.
You're now ready to use the modified board. On first power up, be sure follow the installation guide from the first linked thread: http://www.mediafire.com/?ty3ln8q1cvk243h