For those of you that cannot get the motor for this or until you can, i just remixed an additional handle, you will need to print the lower gear from the original model (the one with internal teeth) for this to be add to, that will then be used as the top gear to be driven like the bottom gear with a hand crank which i have redesigned from scratch using this helpful bearing design https://www.printables.com/model/17916-printed-bearing-608-no-balls-no-assembly. This was all done in tinkercad combining free remixable and commerical use design from LazaroCreations to create a neat free solution to drive the marble run.

I cannot show the original marble run in any of my images for licence issues, but in terms of installation this is to be fit as the top gear in the marble run to enable it to be manually controlled.