Restoring SQL_ASCII dumps to a UTF8-encoded database
I suspect your database content may be in iso8859 or cp1252.  If it were ascii, you would not run into problems importing it.  You may be able to determine the coding by opening your dump with python.  The following python3 tries encodings until it succeeds.  It can be used to determine the file encoding. 

for enc in ('cp1252', 'utf8'):
    print('Encoding', enc)
    try:
        file = open(fileName, 'r', encoding=enc)
        return file.read()
    except Exception as e:
        print(e)
        return None


Python also can also be used to access the database and audit the data.  It could also be used to transcode the data to UTF-8 while copying it if necessary.