How to adapt a non-responsive layout?

Question

css

Good afternoon.
There was an order to adapt the old site. The layout structure is terrible. According to the design that the existing layout has, it is impossible to adapt without resorting to moving blocks through js at different resolutions and duplicating content (for example, the menu on the desktop will be completely different from the mobile one), so you will have to hide some blocks on the desktop or on mobile (the same menu for example). The question is ripening: "how can such duplication and manipulation of content affect SEO?".

Answer 1

If you forget about old browsers, then grids will save the father of Russian democracy.
Or create a mobile version of the site

Answer 2

Moving blocks, changing menus is fine. About duplication is not clear what it will consist of. Hide what blocks? If the text, then this is not very good, especially if there is a lot of text. But hiding the text of the menu and opening it by clicking on the menu symbol is normal.

Answer 3

Isn't it easier to style them and if so change the location of the blocks through order(flex) ?