Very nice design.

Yes, you could use a marble. I have put a glass marble on the sculpture and it does work. There are several problems it presents, though. Marbles are not a uniform size nor weight. A printed plastic marble is of uniform size or weight. For example, a 5/8" marble can vary from about 10% either way or more. That size and weight variance causes problems for regulating the speed of the marble down the track. Also the sculpture is light weight. The heavy glass or metal marble moves the sculpture around while it goes down the track and around the curves. The plastic one does not have this trouble. So since you won't know the size of the marble, nor the weight of the marble, you can't know how the track will behave with the unknown marble. If you print the marble, you can better know how it should behave on the track and have better reproducibility. For this short of a sculpture it probably doesn't matter but for anything longer you can either have a run-a-way marble which leads to a marble that flies off the track.