You already learned your lesson hopefully. If you can do a head gasket, then you can do an inspection to see what's cracked and what isn't. I'm having a blonde moment, so I can't think if it would be a head that would crack. Anyway, if it was a head, I'd just pull one off a junk car. There are members that literally always have cars that they are parting out, so if I couldn't find a junk 525i, I would look there, however they might prefer to sell an entire longblock, rather than taking that apart.