I may be able to help you. I have more than 20 years of Clarion experience with a good chunk of that maintaining and extending existing code bases.
Do you have all the source code? Do you know which version of Clarion did he use?
Feel free to contact me: [email protected]
Sorry I am not connected with the job posting, I just noticed it on upwork and re-posted here. You will have to contact them via the upwork site I think.
Thanks for letting me know.