You will have to remove the powerhead to replace the shift lever. There is a lever on the inner end of the shift lever shaft that has to be removed in order to pull the shift lever and shaft out of the hole. Be careful with that inner lever. Once you remove the screw from it, it is fragile and breaks real easy.
Order a new powerhead gasket and new 303355 gasket and 303347 o-ring for the seal assembly.