I love ornamental sweet potatoes too. Usually I just take stem cuttings (4-6 inches long) in early autumn and remove the lower leaves, and plop them in a glass of water in bright indirect light. Mine usually root within two weeks and make fine little winter houseplants to propagate or plant out in spring.

I've also dug up the potatoes and stored in a cool dark place indoors to replant in spring. That has worked pretty well for me, but not as reliably as the stem cuttings. There really are some amazing varieties out there these days so I like to keep my favorites from year to year.

Let us know what you try and how it works out.
Take care,
Grant