My '03 R53's engine is shot, due to a blown head gasket and/or a harmonic balancer that disintegrated... both problems within a month. The crank isn't repairable, #1 rod is ruined... etc.

I've found a complete engine... S/C included with less than 50,000 miles on it, buts it's out of an '05 R53. Does anyone know if everything is compatible? The ECM particularly.

 

We just did this exact swap in our '03 MCS which threw a rod at 66K. Don't you just love the reliability of these engines!
The swap is plug and play, as they say, with all items being compatible. Before you swap do some of those service things that are difficult at best once installed such as supercharger oil service, water pump, tensioner etc.
Definitely easier out in the open.

Good luck,

Tom in PA