That are two things you are talking about.
You can encrypt (or not) the http traffic for the controlpanel. But therefore it is irrelevant which port is used. So this means, moving this from port 81 to 80 does not necessarily change the fact of encryption.
The other point is, that only one service can listen on a port on an IP-Adress. Assuming that your server has only one external IP adress, you can not configure the ispconfig's dedicated apache to port 80 unless you move your Apache webserver for the "normal" web access to a different port.