Engine+gear box will be a bolt in affair, I'm unsure about the prop shaft. Run N54 DME+CAS(and engine harness) and key set plus locks barrels from a 135i. You would need to get the car programmed, I suspect the JBE and DSC will be ok.
Would be advantageous to buy a 135i that had been rear ended so you could fit all the front end items(cooling system, intercooler etc) too.
This conversion would be costly and could turn into a shit fight without dealer resources. I havent personally looked into this conversion, but there is no reason it wouldn't be easy enough to achieve with some forward planning.