I just installed SME Server 7.0 as ServerOnly and have change port 80 to 81 in httpd.conf
You'll also want to read about "templating". You'll get very frustrated manually changing config files, then having your changes disappear the next time you reboot...
http://no.longer.valid/phpwiki/index.php/Architecture%20overviewAlso, the firewall is only allowing specific inbound traffic - even in "ServerOnly" mode - and is probably denying inbound traffic from non-local networks. If you haven't modified the firewall to allow traffic on port 81 then your website won't work from off-site on this port...
I see there is a configuration database entry for TCPPort for the "httpd-e-smith" service - but I suspect you'll need to do a lot of digging to move your entire webserver from port 80 to port 81.
What about an address translation rule on your firewall (re-map traffic on port 81 to port 80 on the SME)? Or perhaps even a firewall rule on the SME box to re-map incoming traffic on port 81 to localhost:80 (after putting changing httpd.conf back to using port 80... which you can probably do by running "expand-template /etc/httpd/conf/httpd.conf", and "/etc/init.d/httpd restart")
Good Luck!