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.