To use the MITM server, you need to edit the server.exe to listen on port 12346. You can do this by opening up the server.exe in the hexeditor of your choice, for instance in HxD. Now you need to find the integer number 12345. In hex, just look for 39 30 00 00. Change this to the number 12345, so you end up with 3A 30 00 00.
To verify that the change was succesful, run netstat -a -b -n -p TCP when you have started your modified server. Now you can fire up the MITM server. The MITM server will accept traffic from port 12345, so you dont need to edit your client.