Hi, I am interested in working on this project. I would first compare the original theme (your version) with your modified files in order to locate the edits. I will then update the theme, preserving your edits. If you want, I can first clone your site and make a test update, just so you are sure that the job will be done properly. I have created more than 50 highly customizes wp themes and I am confident for this job.
Let me know if you're interested.
Thanks for your consideration,
Bori