The thing is ecu is in back of car in trunk and one needs complete loom. if you get a front cut of BMW motor. that wont solve the problem. you have to buy seperate ecu and loom .
secondly matching a gearbox is an issue. 316i transmission wont run be some big engine because gear ratios are too short, you will have very less top speed and too much torque. engine will rev up too quick and car will have tractor like gears. wont go fast but can pull a bus easy
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so motor plus transmission plus loom eu is a must before you start off swap