Do you need a helmet on a 49 cc motorcycle?

If you're asking if a helmet is required by law, it depends on the laws in your state. For example, motorcycle helmets are not required at all in Texas.

If you're asking whether you should wear a helmet for safety, the answer is a resounding yes. On all vehicles where your body is exposed, including bicycles, you need a helmet. I had a friend killed riding a small motorcycle -- he went down and his hit head on a curb.