Moving an old website with amazing SEO from a no longer supported platform (Question)

Basically someone needs a website moved from adobes soon to be deleted adobe catalyst to WordPress. The site is so old it doesn't have a single responsive element. I'm worried that if I create a site with matching meta tags, heading structure, content structure it still won't be enough. Is it a problem if I do all those things but improve on the responsiveness? Am I overthinking this? I basically told the client I can do it with all the best practices in mind but there's absolutely no guarantee SEO wont be impacted to a degree.

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This website ranks #1 for many keywords with 0 paid seo at any point (its REALLY old!). Happy to provide more info if needed