Rather than change the server OS why don't you change Rumpus instead? By default Rumpus' Web based FTP portal (what you should be using) listens on port 8000. This makes sense as you would not want it to conflict with anything else being hosted on port 80. It's been a while but IIRC Rumpus' interface does offer secure http via port 8443. If not the interface allows easy configurable options for the port suggested or any other port you like.
Thx all of you for your replies. The Rumpus FTP server is has a webinterface and it's this webinterface that interferes since we really want both of them to use https.
I found the solution at the rumpus FAQ and I wanted to share it. You never know if somebody wants to do the same thing.
The solution is to configure Apache as a reverse proxy: