JB Weld or similar product might fix crack in water jacket. A replacement water jacket plus a check for blockage in cooling water circuit would be my preference.
The modular construction of the early K Model Mercurys and Wizards make repair a doable option
even if they have a serious corrosion or wear issue.
If you decide to work on them feel free to contact me.
Currently I have a prewar KB-1A and a post war KD3 torn down for rebuilding.
80% of time and effort is getting motor disassembled and parts cleaned.