Edit /server_root/plugins/snmp/magt/CONFIG to specify the desired port.
Run the start script as follows:
cd /server_root/https-admserv
./start -shell /server_root/plugins/snmp/magt/magt
/server_root/plugins/snmp/magt/CONFIG
/server_root/plugins/snmp/magt/INIT
The master agent will then start on the desired port. However, the user interface will be able to detect that the master agent is running.