(Thank you @Onkel Neal)
I did a little looking, and I found a video showing an original U-Boat U-505 piston, with text stating that, yes they did carry spares, and they could be swapped at sea.
I haven't got a good look at a U-Boat engine, on it's own, but I've noticed that some diesels have a squarish access panel just above oil sump, which would be my guess as to how they got the piston and connecting rod out, as pulling a oil pan on any engine quite a feat.
You can stop the video here and there to read the text. I am not sure of the size, but if I knew the piston diameter, the rest would be easy. Well, it could be resized later, as long as it's in scale. In the video the some rings appear missing.
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