No, modife client dosen't require a port change. As long as both the server and client exe contain the same check sum for the verify section of the code inside the exe then it can be connected.
Unfortunatly, due to the fact that we don't have the source of both the server and client (atleast not that i know of) the only way to do that is by searching in hex with try out and error. Things such as changing a skin, adding custom items by replacing an item ID, etc... doesn't alter that section code at all, thats why you can connect without problem with those exe files.
Up till now i believe (well.. atleast up to last christmas) no one in the community even the japs and chinese has yet to locate the memory address for the port inside the client exe, the only difference between them are the actual different version released by official.