The engine originated as the 2.0 diesel in 1956 (or 1955 for testing), but a petrol version was planned all along.
The petrol version, which had pistons running in the block instead of wet sleeves, and was 2.25l, appeared with the Series 2 in 1958.
With the Series 2a in 1961, the diesel increased in capacity to 2.25, and ditched the sleeves. In fact, there were a lot more changes, and a lot of these also affected the petrol engine, as the two engines now shared as many parts as possible. This sharing includes the block, which meant the crankshaft dimensions (if not metallurgy and finish) of the two had to be the same.
The engines were subject to ongoing continuous development throughout the rest of Series 2a and 3 production, with the major change being the introduction of five main bearings in late Series 3 production (not many of these, if any, were sold here - Australian production seems to have had a lot of three bearing engines to use up).
The engines are completely interchangeable, and so are many of the parts, although not all; for example, the five bearing engines went metric.
So any four cylinder engine from Series 2 to the end of Series 3 is a bolt in replacement (except for early 1958 88 Series 2, that used the Series 1 motor).
With a legal disclaimer! From the late 1960s, increasingly strict emissions controls mandated by ADRs meant that there were various modifications to the engine accessories. It is unlikely that you would be picked up on any of these, however, but it is wise to be aware of what the vehicle should have. And avoid having a mixture of accessories - for example, the first change was positive crankcase ventilation - if you have partly this equipment and partly open crankcase ventilation, you are likely to end up eith no ventilation, which is likely to shorten engine life and may cause seal failure.