If it's not where a mount is or an area that gives support, you can use jb weld. A really good product that's easy to work with and sticks to aluminum very well is called Kop Coat. Bnoon pointed me towards the stuff, and I've used it for plugging the old injector pathway in my oval port heads, closed off and smoothed the IAC area on my UIM, ect. Great stuff
If you have the piece of your trans, you can have it TIG welded back into place. I'm sure it'd have to be cleaned up a bit with a grinder, ect to get it to weld in right. But that'd be the "best" way to repair it.
If it is a mount area or the like, you might have to just get a new case all toghether.
(Insurance might cover it
I had the sunroof rip off on the freeway on my Grand Cherokee, and they coverd the replacment under "act of god"
)
