I've built mobile sites from scratch and additionally have modified an existing one to be responsive. I can come up with the design you could approve or you could give me a design of what you want and I could do it that way too.
I've used jQuery and HTML5 a significant amount, although when adapting a site to be responsive, I use javascript as a last resort, first using CSS. I'm experienced in SCSS/SASS as well, so if you'd like it to be organized more efficiently than normal CSS I'm certainly up for that task as well.
Feel free to contact me by emailing me or to check out my website www.TomPrats.com.
Thanks a lot, have a lovely day.
-Tom