You should be fine in a sun shirt as long as it has a collar and long or short sleeves (standard jumper attire). But check that the prize list does not specifically state that proper or formal attire is required (both require a jacket). This usually only specified for classics or evening classes, but occasionally shows that attract a large public audience or are trying to provide a finals or championship feel will request jackets for all classes. For those shows there’s also sometimes a requirement in the prize list that you ride for ribbons.
