mor, how on earth would a 4 cyl manifold fit on a 6cyl engine? LOL
Anyway, I have lots of doubts as to whether it is cracked, your manifold is most likely stainless steel like the E60, and diesels run so cool, it is not very likely for the exh manifold to crack. But I would first make sure of what the problem is, you will have to do a lot of labour to get it off, maybe it is a gasket, maybe exh manifold, maybe turbo that is cracked. If it is gasket, just replace and all will be good again. If exh manifold, just get one from a scrapyard, if yours is different than the E60, try to get one from an E60, it is slightly better, and will lower turbo lag.
Are you currently running any mods? Do you still have the diesel particulate filter in the exhaust? If so, remove it at the same time, it will also reduce lag as well as exhaust gas temperatures at the turbo and manifold dramatically.