Hummingbird 25mm Motor Adapter is an example of how to create adapters for different motors for Hummingbird (https://www.instructables.com/id/Hummingbird/)
This adapter happens to fit a motor found in my "box of motors" box, a "30RPM DC 6v .1A 25mm Diameter Gear Box Speed Reducer Motor" (still available on e-bay, I just ordered a few more). Being slightly slower than the original motor while having greater torque, this motor is much quieter than the original used in the Hummingbird model.
I've included all files, including the Autodesk Fusion 360 .f3d file containing the design, to allow others to create motor adapters for their specific needs.
To assemble and install this motor adapter, as shown in the four cad images, press the gear on the motor shaft, press the gear / motor assembly into the housing, then press the entire assembly into the base on whichever side you choose (the final image shows the motor adapter assembly installed on the left side of the base) aligning the gear as shown.
If you have any questions, feel free to ask!
Designed using Autodesk Fusion 360, sliced using Cura 3.2.1, and printed in PLA on an Ultimaker 2+ Extended and an Ultimaker 3 Extended.