Parts.

I acquired the following parts:

One N20 6VDC 30RPM gear motor.
One twin AAA battery case with switch.
Two AAA batteries.
Four 12MM diameter by 2.5MM thick neodymium magnets.
One 1/2" diameter "nylon solid plastic bearing balls G1 precision".
One 200CM by 120CM by 1.5CM thick marble slab (actually, I "borrowed" my wife's marble cheese board but don't tell her!).
I 3D printed the following parts at .15mm layer height and 20% infill in PLA unless noted otherwise:

One "Arm.stl", .15mm layer height, 100% infill, Tough PLA.
One "Axle.stl".
One "Base.stl".
One "Loop Rail Left.stl"
One "Loop Rail Right.stl".
Five "Track Clip.stl".
One "Wheel.stl", .15mm layer height, 100% infill, Tough PLA.
This is a fairly easy 3D print and assembly model. Prior to assembly, test fit and trim, file, sand, polish, etc. all parts as necessary for smooth movement of moving surfaces, and tight fit for non moving surfaces. Depending on you printer, your printer settings and the colors you chose, more or less trimming, filing, sanding and/or polishing may be required. Carefully file all edges that contacted the build plate to make absolutely certain that all build plate "ooze" is removed and that all edges are smooth. I used small jewelers files and plenty of patience to perform this step.

The model also uses threaded assembly thus an M8 by 1.25 tap and die will assist with thread cleaning if necessary.

Assembly.

 To assemble the model, I performed the following steps:

Turned off the battery case switch.
Installed the batteries in the battery case.
Soldered the battery case wires to the gear motor such that the gear motor rotated clockwise as viewed from the motor shaft end of the gear motor when power was applied.
Secured the wires to the rear of the motor using a wire tie as a strain relief.
Pressed the motor into the motor pocket in "Base.stl".
Pressed "Wheel.stl" onto the motor shaft.
Pressed two neodymium magnets into the magnet pocket in the base assembly flush with the front of the model.
Pressed two neodymium magnets into "Arm.stl", flush with the side facing the base magnets and matching the base magnets polarity.
Positioned the arm assembly into the base assembly then secured in place with one "Axle.stl", making certain not to over tighten the axle and that the arm would rotate and snap back smoothly.
Pressed "Loop Rail Left.stl" into the rear rail pockets in the base assembly.
Pressed "Loop Rail Right.stl" into the front rail pockets in the base assembly.
Pressed one "Track Clip.stl" onto the top of the loop rails, then secured in place with small dots of glue.
Pressed a second track clip onto the start of the loop rails 30mm above the top of the base, then secured in place with small dots of glue.
Pressed and secured with glue the remaining track clips in the positions shown.
Secured the model and battery case to a flat, level surface (my wife's marble cheese board) using double sided tape.
With assembly complete, I placed the nylon ball bearing in the start of the track, turned on the battery case switch, and watched the bearing loop!

And that is how I 3D printed and assembled "Marblevator, Loop 2".

I hope you enjoyed it!