Grease cake tins with vegetable oil, line with parchment, and preheat oven to 170°C.
Mix pineapple and sultanas in a bowl, and combine dry ingredients with grated sweet potato in another.
Add oil, eggs, and vanilla to the pineapple mixture, then stir into the dry ingredients.
Divide batter into tins and bake the large cake for 55 minutes and small cake for 45 minutes.
Melt marshmallows with water in the microwave for 2 minutes and stir until smooth.
Incorporate orange food coloring into the melted marshmallow mixture.
Mix in one-third of the icing sugar to create a firm rolling consistency.
Knead in remaining icing sugar on a dusted surface for 10 minutes then chill.
Beat margarine with icing sugar and passionfruit juice to make buttercream and chill.
Whisk egg white and icing sugar together until thick for royal icing.
Cool cakes in tins for five minutes before transferring to wire racks.
Shape the large cake into a guitar body, fill, and coat with buttercream.
Cut the smaller cake to form the neck and headstock, then fill and coat with buttercream.
Assemble all cake pieces on the presentation board to form a guitar shape.
Roll out orange fondant, cover the entire cake, and trim the excess.
Divide remaining fondant into three parts and color them brown, silver, and white.
Apply the brown fretboard and use silver and white fondant to create guitar details.
Pipe frets and strings using royal icing and decorate with silver balls.