yes the head will work. the port size has to do with valve size. you can put valves as big as 30mm In and 24mm Ex in the head. if you are going 54mm bore make sure that you deck the head and cylinder to bring up the compression.
the zonger head uses a 28t cam sprocket so be carefull as the gpx is 32t so you will have to grind out the cam chain channel to fit the 32t sprocket in the head.