Assuming you can't somehow claim SULEV or an extended warranty, minimum replacement cost (if you have all tools and DIY) is $4k and 20+ hours. If you take it to a pro, be ready for $6-10k. Not sure what you consider "high mileage", but 4-cylinder X1s at 150k+ miles are worth $5000 at most. So, engine replacement is basically a guarantee to cost more than the car is worth.
If you're really that concerned about reliability, either do some maintenance or sell the car before something goes wrong. There's nothing wrong with putting the cash towards a newer ride. Now if you wanted to swap a different engine into it... that might be worth your time
Link to N20 replacement thread from the F30 page
https://f30.bimmerpost.com/forums/sh....php?t=2039768