This job is divided into 3 parts:
1) WordPress Theme customization - with small HTML fixes on the website, addition of few WP elements and replicating some HTML parts to work on other pages.
2) WordPress Theme optimization - As per Google and other tools, site needs to be optimized to work better on Desktop and Mobile devices. For this, we need to play with the CSS and HTMLs to make it more user friendly. (Will attach the document).
3) Htaccess and XML implementation - this requires masking URLs from the htaccess and checking and fixing robot.txt and XML sitemaps.
I would furnish more details later (specific about website) once you read the above and tell me (in your response) what would be your approach in doing the above.