Wow, never had that problem in 200,000 + miles.  The studs/nuts must be
loosening, that is the only way I can figure that the studs or brackets
would be breaking.  If they stay bolted up tight they are plenty strong
for the 90 amp alternator.

Helicoil is the solution if you can get a straight on drill path for
installing it and the hole is not too big now.  I'm surprised you are
not leaking coolant, these holes are sometimes drilled right though the
water jackets.  Use the right type of stud with a center band that is
not threaded so you can firmly seat the stud in the case (use the two
tightened nuts on the stud method), and use Locktite as well on both the
stud and nut.

I don't like the idea of cutting slots like your mechanic did, but there
should be plenty of metal there and that should not be a problem.

Is it time for a rebuild yet?  Jerry at NW Con Rod in Seattle welds
these cases and inserts Helicoils to fix problems like this when he does
his rebuilds.  He might even work on yours in-situ, so you might want to
give him a call.
--
Stuart MacMillan
Seattle

'84 Vanagon Westfalia w/2.1
'65 MGB (Daily driver since 1969)
'74 MGB GT (Restoring sloooowly)

Assisting on Restoration (and spending OPM):
'72 MGB GT (Daughter's)
'64 MGB (Son's)

Stripped and gone but their parts live on:
'68 MGB, '73 MGB, '67 MGB GT