How To Insert String With Special Characters Into RTF

How to programatically insert string with special characters into RTF? I have rtf template I load to string and then replace all $MY_VARIABLE$ with data. Data contains special chars like 'ąęść' and the problem is that in result file these characters are replaced with '?'. It's something wrong with encoding but what?

My code looks like:

StreamReader reader = new StreamReader("template.rtf");
StringBuilder form = new StringBuilder(reader.ReadToEnd());
// here I replace variables in rtf with data
Encoding srcEncoding = new UTF8Encoding();
Encoding dstEncoding = new ASCIIEncoding();
byte[] utf = srcEncoding.GetBytes(form.ToString());
byte[] asci = Encoding.Convert(Encoding.UTF8, Encoding.ASCII, utf);
return dstEncoding.GetString(asci);