Regarding the text encodings understood by mkii and mkiv, one can always use UTF-8 in both.Unless you have some very uncommon characters in your ascii file, the migration from ascii to utf-8 is quite easy, at least on a Mac (I don’t know what OS you are using): for instance you can open the file in an application such as Smultron or Fraise
and then in the « Text » menu choose « Text encoding » and then « UTF-8 ». After that you save your file: now it is in UTF-8 encoding and can be typeset in mkiv, as far as the commands you use in your TeX file is compatible with mkiv.