Officially, in SD, under 50cc and without requiring shifting is a moped, regardless of HP or top speed, and does not require titling or registration. You also need to have:
A valid driver's license.
A headlamp "of sufficient intensity to render clearly discernible a person or vehicle at a distance of not less than one hundred feet.
A "stop lamp" (brake light, not just a taillight) visible from a distance "not less than one hundred fifty feet."
If you are under 18, you must also wear a helmet.
Go here and search for "moped":
South Dakota Codified Laws Text Search