Replace EOL in stream
Performance


The non-Async methods are much faster in case of ReadLine/Write. If you need to run the code in background, use await Task.Run(/* put the whole code here */);


Encoding


You can use the static property Encoding.Unicode. (It is the same as creating a UnicodeEncoding instance without constructor arguments)
Add the Encoding to the stream reader: new StreamReader(fileInfo.Open(FileMode.Open, FileAccess.Read), Encoding.Unicode). Otherwise, if the input file is not unicode encoded, the output becomes a bunch of trash.