Can I use for my AMP pages for Google meta charset and page encoding NOT in utf8; but windows-1251 for example?
I can't find any info in the specification about it.
You cannot use anything else than UTF-8 for the meta charset, as noted in the AMP HTML Specification:
AMP HTML documents MUST
... contain a
<meta charset="utf-8">tag as the first child of their head tag
Possible solutions to converting your content's encoding depend on the server technology you are using.
You can set the charset to anything you want:
However, know that users using a different character set will see a lot of garbled characters in your pages. If your website is 20 years old I would say it is time for a facelift and a few tweaks. You can find online utf-8 encoders that will make it easy for you.