It's still ugly though

private static String BackwardsNumCheck(ulong Num, String src)
{
    Num = Num > 9 ? '9' : (char)Num + '0'; // could use a Max() here, if the platform has one?  In C it'd be a macro...
    int len = src.Length();
    int i;
    bool carry = false;
    String ret = new String(' ', len); // I hope I can treat this like an array.  String classes are ick.
    for (i = len; i--; ) {
        ret[i] = src[i] + (carry ? 1 : 0);
        if (carry = (ret[i]++ > Num)) {
            ret[i] = '1';
        }
    }
    return ret;
}