As stated the engines are interchangeable, although early engines had different radiator hoses.
As suggested, there are possible issues around the clutch. If you are using a 2a gearbox, use the 2a clutch. The standard 2a clutch has three fingers, and uses a pressure plate with coil springs.
It was replaced optionally (and standard later) with a slightly larger one using a diaphragm spring and a lot of fingers, similar, but not identical to the Series 3 clutch (the driven plate is the same as the S3). The difference in the pressure plate for the S2a is that it has a spacer between the fingers and the release collar, held by three flat springs to the frame of the pressure plate.
The bolt holes and spigot in the flywheel are, I believe, the same for both engines.
John
JDNSW
1986 110 County 3.9 diesel
1970 2a 109 2.25 petrol
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