To answer very specifically your question: yes, not having the original engine detracts from the value, all other things being equal.
What I mean by that is that the condition of the engine, original or not, is similar. So not "original engine completely worn out with barely any compression needing rebuild" vs "replacement engine, recently completely rebuilt by Metric Motors and in new condition mechanically and cosmetically".
Btw all these engines are factory original.
My personal opinion is that it's the condition of the engine that is important, not the serial number that it has. It's also very easy to fake an original engine number. And, many engines lost their little aluminum plates along the way as they were not riveted onto the block very solidly. So, the one you are looking at may still, or may no longer, have the 'tauschaggregat' plate. If that is important to you, you can just get a new one and affix it to the block.