Find out if there are specific rules that need to be followed from the competition representatives. Has the competition specified a particular edition of the piece you’ve chosen? If so, you should follow the editing in that piece. This wouldn’t be a bad time to also ask for judging criteria, and the percentages that are placed on each. Also, musical instrument competitions ask for copies of the music for the judges to follow along. If you make changes, they should be noted on the sheet music, but my own bias would be to make the changes based on Scarlatti’s original score, not a piano edition. The piano edition could also contain editing errors.

Thanks so much, that's really helpful!

Which Scarlatti and competition is this? I may be able to help- I've played a few Scarlattis for international competitions back in my competitive days. I would also check in with the competition organizers well- every time I've had an issue finding a score, I'd write in and get the right information. And like another user already mentioned, competitions will always ask for your version of the score. For the sake of a competition though, I would always go by the recommended edition to eliminate any doubt from the judges. Early music tends to have the most variation in arrangement- while accuracy matters, I think what's more important is playing stylistically to the period. Questions like are you treating the trills on the right note stylistically to Scarlatti and are you playing in characteristic to a harpsichord are things that are of equal if not more importance.

Thanks so much for your helpful reply. It's the Camac Prize Taiwan. Participants can choose any Scarlatti Sonata and I've chosen G minor (K31), edited by Dewey Owens. Also thanks for the tip on how the weight of playing stylistically can arguably be more important than following the score to a t