Porting VNs...AHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHA.
There is no porting VNs anywhere. It's either you get it legally, get it illegally, then tear it to shreds with whatever extraction tools that may or may not be available for said game to rip out sound, music, visuals, text, etc in some form that is hopefully actually convenient to use, and then either use your masterful coding skills to parse the shit you extract to convert the shit from ONScripter, NScripter, KiriKiri, Ren'Py, RealLive, N2System, and literally fucking anything else under the sun OR you manually parse the shit yourself by hand to an engine that you want to use. The more text there is, the more annoying long the process gets. The more choices there are, the more chances your bullshit script is going to fuck up one of those choices and you won't find out until you hit the route.
It's almost as if you are expecting all VN engines to just be the same, BUT there were at least 167 different VN engines back in 2009 and that wasn't even dealing with the random VNs that could be using who knows what.
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Also, if you still want to deal with that shit, VNDS script stuff was made for the retarded. It has next to no cool features, but at least it works and you don't have to think to hard about what you are converting to since aside from images, audio, and text, VNDS doesn't support it.