There is a plug on the port side under the powerhead -- pn 307581 or similar. It looks like a lower unit drain screw. The plug is at the top of the water tube. If not in place, water will shoot out of the opening rather than circulating through the engine. If that plug is missing or loose on your engine, assuming you have it on your model, the motor could overheat. Think I would first check for the plug.
The tank could be patched with Marine Tex or something similar. Depending on how caustic the cleaner is, you might have some other leaks but likely repairable. And you can find used tanks if it comes to that. The tank was used for many years.
[edit. See the plug on your first pic in post 29 -- so apparently have one in place. See if it's loose.]