The bolt has probably broken off because steel & alum do not like each other......
They corrode & we know the rest of the story....
Luckily, the offending bolt [ now a stud ] is [a] easily accessible & has very little load on it. It is sealing a gasket against coolant pressure.
If you cannot remove the stud or do not want to try, then I would drill & tap a 3/16" hole either side & close to the stud. There should be enough metal to do this. Use grade 8 or s/steel bolts to lock the housing down.