You can find the delimiter, convert the substring of the bytearray to str and then int. Here's a little function to do that:
def intToDelim( ba, delim ):
i=ba.find( delim )
return int(str(ba[0:i]))
which you can invoke with
value = intToDelim( data[8:], b'\x04' )
(or with b'\x03' if that's your delimiter). This works in Python 2.7 and should work with little or no change in Python 3.