As to the topic of R45 and R65 cylinders & head compatibility
Both R45 and R65 models of the same vintage (either pre-Nikasil (< 1980) or Nikasil (1981-86(( had the same 61.5mm stroke, but the
R45 bore was 70mm, whereas the R65 bore was 82mm. That half an inch diameter difference up at the head could have issues. Along with this, the R45 head uses smaller valves (In 34mm, Ex 32mm) versus the R65 (In 38mm Ex 34mm for pre 1980, In 40mm, Ex 36 for 1981-1986) so you really do need to swap heads for the larger R65 valves to get the benefit. Even if the 12mm "step" in the R45 heads can be worked around, the smaller valves will dramatically reduce the power gain achievable with the R65 cylinders in place.. But, at least the connecting rods are the right length (I am assuming the wrist pin diameters are the same - maybe they aren't)?