The fischertechnik mini motor comes with a pinion and gear adapter. Unfortunately, the smaller Z11 gears of this adapter are prone to break sooner or later. Proper replacements can be ordered e.g. from the great fischerfriendsman.de site, but I was curious if you could also print them. My tests came out fully functional, but I wouldn't want to bet on their durability.
Hint: One of the gears in my images is an original from ft. ;-)
Find my other fischertechnik designs here.
A perfect first layer helps to keep friction at a minimum.
You might want to use a 2mm drill before installation to ensure that the gear fits on its axle. If your first layer is not perfect, as in some of my samples, use a small file to remove any excess material between the teeth.
See ft:pedia 4/2011 pp. 23-25 for instructions on how to (dis)assemble the pinion and gear adapter.