Tim, despite what D.Valentino said ALL Sprites and Midgets were positive ground until around October 1969.
That said conversion to negative ground is really very simple, i.e., switch the battery terminals (natch), reverse the two wires that connect to the coil, and re-polarize the generator. No new parts required. Sometimes, but not always the two wires to the heater fan motor will require swapping for the motor/fan to blow instead of sucking. To repolarize the generator run a temporary wire from the positive battery terminal to the generator and swipe/spark it on the "F" terminal, then reconnect the generator wires and it's done.