I wonder if the piston will travel the full length of the R80 cylinder because the stroke is different compared to the R45.
The R80 stroke is 70, 6 mm, so the final bore and stroke is the combination of R80 bore and R45 stroke. R80 cylinder is indeed longer, so without machining the skirts, it won't be able to crank in the case.
If I understood the focus of your question.
Edit: I think you question is dawning on me, and I'm not sure what the answer is. If I understand your question right, it would not be enough to know the volume of the head, but also the length of the R80 cylinder that the piston doesn't travel (because that volume would be part of the combustion chamber in the short stroke engine, but not in the long stroke). I don't think it's really that much, because the valve recesses in the piston had to be machined to be deeper, to prevent valves from the R65 head from hitting the R80 piston.