You can go to the "Mii characters" menu on the title screen, go to the Petey you have (or download another from the same key if you want to keep the original as well, then choose that Petey), then edit the Makeup.
Add/Remove/Move the makeup parts as you wish to make him look like Li'l Petey. Use the "Edit Mii" option to change non-Makeup stuff, like character size.