The challenge this time is the motor shaft to gear connection.
The original design uses metric pieces, which are available in Europe but not locally!
The design uses a Module 1.0 rack with the 12 tooth gear. You don't appreciate how tight those tolerances are until you order a few "I think this will work" pieces from eBay and wind up with a stack of parts that won't work!
Add to this a 12 volt, 300 RPM motor with a metric shaft diameter of 6 mm.
So, we jump into our arithmetic (that's "math" for you kids!) and determine that 6 mm is 0.23622 inches. This number comes in handy in a moment.
The solution to not being able to purchase the Module 1.0 brand rack and gear was to buy something similar from McMaster Carr. I purchased the 20 degree steel rack and two 15 tooth gears to go with it. I shipped the steel rack to a local machine shop to cut them down to specified lengths and have the holes and grooves cut. Once back, those were attached to the brackets.
The gear bore has a 3/8" diameter and the motor shaft is 0.23" I was able to find a bearing that has the outside diameter of 3/8" and an inside diameter of 1/4 inch. Which is really close, 0.25" versus 0.23"
My approach is this...drill and tap a 10-24 thread hole into the gear, drill a slightly large hole in the bushing, then use a set screw to screw the set screw into the flat side of the motor shaft. That should help tighten up some of the 0.02" difference
And here's it is in action..