A new gasket is always a good idea but you may get by with one of the used ones if one is still in good condition. The water tube connection would be the main concern. I wouldn’t use a chisel. Something with a blunt end would be better. If you have a spare wrench to grinding down the box end to fit, that would be the best option.
A "Boathouse Repair" is one thats done without having tools or the skills to do it properly.