The first thing you need to know is what encoding the files are using. ANSI is 8-bit (0 to 255 character codes), ASCII is 7-bit (0 to 127 character codes, though many times nowadays it's 8 bits as well, with the leading bit always being 0), then there's UTF-8, UTF-16, UTF-32....
Programs like Notepad will try and guess at the file's encoding -- the first few bytes should usually be a header which tells you what is it, exactly -- but that header is often missing or wrong; in which case you just need to "trial and error" until the text makes sense, unless you know the encoding and can enter it manually. (Which is why notepad has the option to choose encoding when opening a file -- no matter how good it is at guessing with its "auto-detect", it can still get it wrong sometimes and need human alteration.)
Only once you know the encoding,
then you can go about converting it down to a standard 128 character values (standard ASCII). To do that, you basically need to get a list of all the code values for that specific encoding, and then map them over to what would best represent them in your new encoding.
http://www.fileformat.info/info/charset/UTF-8/list.htm -- UTF-8 encoding codes can be found here, for example.
Looking at the chart above, we see that "è" is C3A8 in hex -- CHR$(195) + CHR$(168). Since we know the text file is encoded in UTF-8, we can now convert all CHR$(195) + CHR$(168) characters into CHR$(ASCI("e")) characters...
But, without knowing the encoding first, you're just blindly altering values, without being certain which ones actually need changing or not. If your files have 2-byte characters, I'd guess them to be in UTF-8 format, and they should convert over to ASCII just by mapping the code page above to standard ASCII values. If they're not UTF-8, then you need to know/detect what they're in, (and hope you don't detect the wrong encoding), and then convert from whatever format they are in, to the one which suits your needs.
The steps to solve this type of problem is:
1) Determine the encoding the file is currently in.
2) Determine the encoding you want to save them in.
3) Map a set of values from the first to the second.
4) Save the converted file to a different name, just in case you screwed up with step 1 or 2, so you don't corrupt your original data.