I think PEP 393 gives us a quick way to fast parsing: if the max char is < 128, just roll straight into normal processing, otherwise do the normalisation and "all decimal digits are from the same script" steps.

There are almost certainly better ways to do the script translation, but the example below tries to just do the "convert to ASCII" step to avoid duplicating the +/- and decimal point processing logic:

    if max_char(arg) >= 128:
        arg = toNFKC(arg)
        originals = set()
        converted = []
        for c in arg:
            try:
                d = str(unicodedata.decimal(c))
            except ValueError:
                d = c
            else:
                originals.add(c)
            converted.append(d)
        if (max(originals) - min(originals)) >= 10:
            raise ValueError("%s mixes digits from multiple scripts" % arg)
        arg = "".join(converted)
    result = parse_ascii_number(arg)


P.S. I don't think the base argument is especially applicable ('0x' is rejected because 'x' is not a base 10 digit and we allow a base of '0' to request "use int literal base markers").