Hi and welcome to the forum.
Corrosion is the main reason the pump could be seized. I wouldn't take the top off unless I had to. drain everything, Plusgas the bores, not too much. Clean the carbs and tank and flush out the water changing the pump if it is definitely seized. try and turn the engine with the plugs out to see if things move around in the engine. new oil, water and fuel. A splash of fuel in each bore, plugs in and see what happens.