First point: ASCII stops at 127. When you say ASCII, I assume you mean Unicode or perhaps iso-8859-1?
Base64 is an encoding of a byte stream. So I assume you want these codes treated as single bytes. In that case, it's actually immaterial whether they are ASCII codes or iso-8859-1 codes or anything else - they are just bytes.
There's no direct way of doing this in pure XSLT 1.0 or 2.0. Saxon however has an extension function that you could use:
should do the trick.