I’d suggest you use that Waterwitch for a parts motor and get a complete one, perhaps with a better tank. I successfully straightened a ‘witch tank once using air pressure (10psi). and propane torch heat and tapping around the dent simultaneously with a wood handle, with the gas cap vent open to allow some air flow during the process, with an air regulator to maintain 10psi.
If you use aluminum solder, don’t use it on your tank until you practice on junk tanks first. Results may not be as expected. The solder is very difficult to file or sand. It is also a different color.