New Beetle 1.6 petrol: AYD versus BFS engine

[Cider Andy]

A friend has a very high mileage 2004 Beetle 1.6 petrol with a BFS code engine that needs replacing. Secondhand BFS engines seem few and far between and they're relatively expensive, whereas AYD code engines are more plentiful.

Does anyone know the AYD engine is a straight swap for the existing BFS engine? Or, if it is, any reason why it shouldn't be done?

[AJB88]

probably is, will need the ECU from it though and will need it coding.