Hey,
Sorry to hear your bad news... T
The engine on the OneD is tough as old boots being a Toyota unit, as long as it is serviced and checked pretty often it should be fine.
I have noticed recently on my OneD that the expansion tank for the radiator has lost fluid. It seems to be seeping out of the cap and running down the bottle.
I am sure I am not alone on this and if left for to long could result in low coolant and then the engine is prone to run hot. I guess this is what happened to you, they replaced the engine but probably not the expansion tank and pipe work...
BMW's dealer work is guaranteed for two years outside of warranty, so if they have fitted a new engine and ignored the item that caused the problem they should be responsible and liable for repair work!
Good luck... | 
