Path of least resistance at this point might be to just replace the water pump housing. One on eBay, or breakers such as Spitfire Graveyard and Spitbitz are worth a call https://www.ebay.co.uk/itm/225089997171?mkcid=16&mkevt=1&mkrid=711-127632-2357-0&ssspo=Ksh36BaPRvq&sssrc=4429486&ssuid=en_MztWYTyu&var=&widget_ver=artemis&media=COPY
If you don’t want to go that route yet, taking the housing off and gripping the sender in a vice is worth a try. Also possible to completely destroy it by drilling it out and using a tap to clean the brass remains out the threads, though there’s a risk of damaging the threads in the casting, or them being so rusted to be of no use.