Hi there, my name is Christopher and I'm an expert in Linux/unix and web hosting security. Also I am very good on PHP and MySQL debug and fix. I would be glad to help you with the update of your opencart website. From my experience i think it would take me maximum 2-3 days of work.
For not having your site down this day, i would suggest you sent me an archive of the current website and i will work locally and after is ready, i will give you a link for testing. After everything is completed, i will move the website to your domain/server.
Best regards,
Christopher