The Living Computer Museum in Seattle has a fully functioning Alto, as well as a bunch of other vintage systems.
I played with it. It was as amazing as I had hoped.
There are a bunch of running Altos, so we're not exactly in uncharted territory. Fortunately people with Alto experience are willing to help out, for example Al Kossow giving us a disk interface card when we found ours was the wrong type. The Living Computer Museum is probably the best bet if you want to see one in operation.