I have a pre 81 model R65 and I think you will need to do more than just change the cylinders & pistons to get the full benefit of the larger capacity. The heads and carburettors are also different part numbers on the R65 to the R45 and there was even a lower power version for the German market.
If you are looking for outright performance from the R45/R65 series there is “proper” 800cc conversion kit available.
But just as flipflop suggests join one of the many airhead forums - there is even a dedicated BMW R65 one which you may find the answer in.
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