Hi there...
I read your project, description and very interested. I have a lot of experience modify 'old site' to responsive site, especially after issue google will ban nonresponsive site from their SERP. I understand your main goal is pass all google mobile friendly test. Believe me I have lot of experience this kind of project, I will modify your site using CSS and bootstrap so you don't need to worry, all of your content will be same. and I will keep you previous design line to.
I am a talented Web Developer with years of experience and educational training.
I able to use PHP, HTML, CSS, JS, WP, Responsive design with Bootstrap, etc.
If you hired me I promise you will not disappointed. I will deliver my best work at a high standard.
Thank you