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 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.