Have no idea why this companies rankings were not demolished, not sure how to proceed.

I was hired by a niche company to take over their website. They currently rank #1 for about 90% of their targeted keywords and are very scared to lose them, rightfully so (obviously)

Their current website is built on a propritary CMS but they own the design so I was just going to copy the design and all the content to have as little risk as possible. More then just making sure the url's, page titles, meta descriptions and content are the same, I am literally matching the alt attributes of images, the exact pixel sizes, etc.

So here is where it gets interesting. They hired a company a long time ago to build their original site which they had for 10+ years. That company however, did not manage it. The company that handled site management just made them a new design a few months ago. This new design looks nice and it does seem they copied the same page structure (although I can't tell for sure because archive . org is down so it's only based on my memory) but there are a lot of very beginner level SEO errors.

Here are some of the most noticeable I have come across in my first 10 minutes of looking:

1) The home page Hero section has their H1 and then directly under that, an H6 instead of paragraph text.

2) A lot of the pages have no H1 at all.

3) Not a single image has an ALT attribute.

4) I could keep going but this is already a long post and I think you get the point

I don't have access to their Google Search Console and won't until the website is finished and they have fully transferred over to my service but I do not see that they lost any of their rankings (yet) in SEMrush.

I believe most of their ranking has to do with all the good reviews on their Google My Business pages, Trip Advistor, Facebook, etc. However, I do not want Google to index the new pages from a few months ago and tank their rankings just as my version of the site takes over.

Should I...

1) Still copy the new site design, even the stupid SEO errors. 2) Still copy the new site design but fix the stupid SEO errors. 3) When The Way Back Machine is back online, get the content from the old site and use that instead.

Thanks in advance for any help!