I started using stepper motors to power LEGO, but they were heavy and really over-kill for what I needed. So I decided to find a cheaper motor that can be used easily to make small LEGO vehicles.
The motors that I found are these small N20 geared motors. I won't post any links here, but you can easily find them on ebay/aliexpress/wish by searching for "N20 motor".
For this project I am using motors with a 10mm axle length. Also I am using a 6V motor with 300rpm.
Any slower than 300rpm would probably be a bit to slow to direct drive a wheel.
Remember to order some M1.6 bolts/screws when you order your motors. The local hardware store probably won't have these... M1.6 x 5mm bolts
Next, I designed the motor mount using OpenSCAD - a free scripting CAD program.
The housing is in two parts - an enclosure for the motor:
And a face-plate to attach to LEGO beams:
The final part is an axle adapter to convert the motor's D axle to a LEGO cross axle:
I had these printed on 3DHubs.com in PLA printed at 100μm.
Here is the assembly process:
Now for the creative part.. Making things!