Summary

This is a simple longer version of chasmaker original smaller stepper gear. This one will not move on the axis and it is very robust.
The picture shows it after 10 month of use and kilograms of PLA. No wear so far! It works very well with my big gear at http://www.thingiverse.com/thing:25501

Instructions

Sorry, no source code in the first place. I used meshlab to extend the gear!
When I printed chasmaker' beautiful design in PLA (instead of ABS), I quickly found that it failed (because of my stepper heat on some tricky prints -- I was having feeding problems at the time).
The benefits of this longer derivative are:

the gear will not melt as with the small gear because there is much more material to heat!
it has a better grip on the axis: it will not move inwards and let the big gear free. You will see on the picture how I added a short piece of bowden tube in addition between the motor and the gear.
it is probably strurdier in the long term than with ABS
Printed with PLA, 100% infill, 0.2mm layer height, slow speed and lowest temperature possible. I also used the "Joris" option in Cura to reduce the seam (aka helicoidal print).
As the name suggests it, I had to scale the object to 1.025 before slicing, otherwise it was impossible to fit on the axis. Gently heat your axis with a lighter and then firmly push-to-fit. Do not overheat or mistreat your stepper of course, better print another gear ;) You should leave no backlash at all of course else it will deteriorate quickly.
Thanks to chasmaker for the original design and to (open sourced) Meshlab authors!

Drop in replacement: ultimaker small stepper gear