You need to go to AppData>LocalLow>Jundroo>Simpleplanes>AircraftDesigns and then find the stock crafts (They should be at the top). Stock crafts are marked with a double underscore on the beginning and the end, and to convert them into crafts that are available in the craft list, you need to copy one of these and delete the underscores.
@RocketPal thank you!
You need to go to AppData>LocalLow>Jundroo>Simpleplanes>AircraftDesigns and then find the stock crafts (They should be at the top). Stock crafts are marked with a double underscore on the beginning and the end, and to convert them into crafts that are available in the craft list, you need to copy one of these and delete the underscores.