There are a few options:
Use a tap and die set to rethread it with a larger size bolt.
Have the hole welded up, drill it out and use the tap and die to make
the same size bolt hole
I have never used this solution, but there is stuff called thread maker
that is basically glue you put in the hole to rebuild the threads.
Probably good enough for this use, because unless I am mistaken, you
won't be putting much pressure on it anyway or you will ruin in the gasket.
>I have a slow leak in the differential in my 1972 MGB. I believe I stripped
>the bottom screw on the diff cover, meaning it is stripped in the diff
>housing. How can I repair this?