With the benefit of a bit longer to think about it...
When the Canadian Lancaster came over a few years ago, one of the Packard Merlins failed. IIRC, they finished the tour and flew back home with a Rolls-Royce engine borrowed from the BBMF.
Apart from propeller splines, which I only think I remember being different, other ancilliaries such as carburettor, dynamo etc. were definitely different so although it probably didn't make any real difference to a pilot, it did to the stores as spares for one engine wouldn't necessarily fit the other.