Writing Text File in Dynamics AX

The function strLfix would be the most likely fix. A similar function strRfix right aligns the text to a specified lenght.
Also remember to set the output field delimiter to the empty string. The default of one blank space is not relevant here.

static void TextIoTest(Args _args)
{
    TextIo f = new TextIo(@"C:\Temp\test.txt", 'w');
    ;
    f.outFieldDelimiter('');
    f.outRecordDelimiter('\r\n');
    f.write(    strLfix("abc",20), '|', strRfix("xyz",15), '|', 117);
    f.writeExp([strLfix("abc",20), '|', strRfix("xyz",15), '|', 117]);
    f.write(); // To force CR NL
}

The job opens the file for writing (truncates existing file or creates a new file), set the field and record delimiters, then writes the same line twice (using two different calls). The last call to write appends the record delimiter, this may or may not be important depending on the receiver.
You can use strrep, substr, and strpoke, please find an example in \Classes\VendOutPaymRecord_UK_BACS\output.