Write non-latin characters and special symbols (Ø) to RTF file (javascript)

Can You please help mу how to encode non-latin (russian) letters, that are mixed with special symbols, for example: Abc Ø абв (here is english text, special symbol ‘latin o’ and russian text).

I have existing RTF template with ‘placeholder’ text inside, and what I need is to replace this ‘placeholder’ with ‘Abc Ø абв’:

I use function from here, at the bottom of the page to decode UTF-8 to Win-1251 – it successfully writes russian letters but finally I get “Ш” Instead of ‘Ø‘:

Here is my example code and input and output files:

input rtf: https://mega.nz/file/CtNB2CiY#yid1nLq9P6Jo8zSRAsXeGai-mZLV6xP1OvN1jDpFyG4

output rtf generated by the code below: https://mega.nz/file/asMExKJI#q8oRn1J9oWMlUck6tJ6MdpVGiIjt81kNFRo7T3eSBTU

const http = require('http');
const port = 3100;

function utf8_decode_to_win1251(srcStr) {
  var tgtStr = "",
    c = 0;
  for (var i = 0; i < srcStr.length; i++) {
    c = srcStr.charCodeAt(i);
    if (c > 127) {
      if (c > 1024) {
        if (c === 1025) {
          c = 1016;
        } else if (c === 1105) {
          c = 1032;
        }
        c -= 848;
      }
      // c = c % 256; // ???
    }
    tgtStr += String.fromCharCode(c);
  }
  return tgtStr;
}


const server = http.createServer(function (req, res) {

  const fs = require('fs');

  // read existing file
  fs.readFile("C:input.rtf", "utf8", (err, inputText) => {
    if (err) {
      console.error(err);
      return;
    }

    // I want to replace 'placeholder' text in file with this test text:
    let text = `Abc Ø абв`; // 'Abc Ø абв'

    text = utf8_decode_to_win1251(text); // text with encoded russian letters 'Abc Ø àáâ'

    // replace placeholder from input RTF with text with non-latin characters 'Abc Ø àáâ':
    inputText = inputText.replace("placeholder", text);

    // RTF uses 8-bit so need to convert from unicode
    let buf = Buffer.from(inputText, "ascii"); // "binary" also gives wrong output text https://stackoverflow.com/a/34476862/348736


    // write output file to disk
    fs.writeFile("C:output.rtf", buf, function (error, resultFile) { // result file contains 'Abc Ш абв', which is wrong..
      if (!error) {
        console.info('Created file', resultFile);
      }
      else {
        console.error(error);
      }
    });
  });
});


server.listen(port, function (error) {

  if (error) {
    console.log(`${error}`);
  } else {
    console.log(`listening on port ${port}`);
  }
})