Yamaha F4 1999 4 stroke cooling problem

Hi, new member here. Recently picked up a 4hp that runs great but has a cooling problem. Impeller and thermostat were toast so i replaced them. Before re-assembling i blew air thru the water tube from the bottom, and i used a cheap pond pump to circulate a water/vinegar solution to dissolve any salt crud from the water passage. Cleaned the thermostat cavity up quite nicely. There was some water leaking down from inside the exhaust tube, and i can hear air leaking up there a bit. Put it all back together but still no water from the telltale or the exhaust outlet in the leg. I suspect the rubber seal at the top of the water tube is rotted or melted. The service manual says to pull off the powerhead to repair it. Basically a full teardown. Has anyone got a method to replace the water seal at the top of the water tube without pulling the powerhead?