Opencart 2.2 - 404 pages fix or new site - solution

Looking for an expert in OpenCart who would help us to get rid of over 12,000 - 404 pages (they are .html dead pages). The reason we have that many as this site is kind of old. We upgraded it few times thus many broken links. We have a good SEO that captures them in but if we re-route them by hand is kind of time consuming to do that many. Currently OpenCart 2.2. - new opencart template, fresh opencart 2.2 - but products were copied from old site and still shows 404 pages - no change just admin looks cleaner. 1. We need to either move that whole site to a new database so it stops with those 404 pages. 2. Upload few extensions, make few minor fixes. 3. Secure the site 4. Check API - (we have it set up on google there but check if it is working). 4. Point to our domain. If there is a better solution that would stop login those 404 pages the lets hear it. Technically it works well but it does not index that well. We want help with technical stuff as later we will SEO that site as well. Currently ranks in 28 spot on any search - due to low backlinks count, plus that technical broken links affect is as well. Kind of urgent - project...