umm . . . i suggest having it diagnosed at another place for a second opinion - if not casa, try autosport beside prestige bmw. naji, the owner is very well rounded with engines and has lots of connections with regrads to parts . . . he even does tune ups for rare cars; so bmw should not be a problem

as for a swap, i don't recommend swapping it with another car engine

for one, your dme will not recognize the engine anymore - therefore, some important sensors may not be functionable anymore

besides, its a bmw . . . the fact that you are determined to keep the car instead of selling it due to its condition shows your personal liking towards it