They seldom "break" off entirely flush with the head (not the block). The gaskets are a laminated steel, and once corrosion gets in the layers, you get a leak. You can't fit the new one in there anyway, without removing or at least loosening the manifold to pull it away from the head. You may as well replace all the studs, as Dan suggested, and all new gaskets.

The studs actually come out better than you would think, as they corroded off rather than broke off from too much torque. The biggest problem encountered is the bolts with the nuts still attached have corroded enough that they often end up a ******* size. I've routinely had to select a socket that is a fraction too small, and drive it on to remove the nut, or sometimes the nut and the stud.

The gaskets and studs/nuts from the dealer are fairly expensive ( and obviously they do not hold up). You can't go too far wrong by just picking up some grade 8 metric bolts/small washers, and aftermarket gaskets, like Felpro.

For easier access to some of the bolts, the inner fender may have to come out. You'll see why after spending a couple of hours on 1-2 bolts. I've always cut the down pipe bolts also, and just replaced them. Without doing that, moving the manifold around much will be limited.

Before you put the manifold back on, check it really well for hairline cracks.

Dave