Alternative for RealVNC?
Chicken

If you are looking for a VNC client, the open source Chicken client supports connecting to specific ports.

Tiger VNC

You can install Tiger VNC via the brew project with:

brew install tiger-vnc


macOS Built-in Screen Sharing

Alternatively, you can use macOS's built in Screen Sharing application to connect to specific ports via a well formed URL, such as vnc://192.168.1.34:5901.

VNC Servers

For VNC servers, other than the built-in macOS VNC server, take a look at OSXvnc.

Tunneling

You could take a different approach and tunnel the existing VNC service through ssh or another tool. This would expose the VNC service through any port you desire without needing a special VNC server.