Oh that's awful. You were afraid this would happen weren't you? Perhaps you could get in touch directly with the clients who bought your work and pitch directly to them. Explain that you wrote the original and you thought you might be able to offer them more? Don't mention the relationship breakdown with the other guy. I'd give upwork a miss. I don't know if you can leave a response to the review, or create a new account, if you still want to work on there?