I have doubts about the 900cc K75;
Technically, you can rebore the block from 67 to 70,5 mm, but no more: the cylinder wall is then too thin (this explaining the oil consumption of the K1100, because of the deformation of the cylinder)
That will give a 819cc engine capacity.
To get a 900cc, it would be necessary to increase the stroke from 70 to 76,8. That exceeds by far the K1200 stroke (75mm) , I wonder how it could be possible to use such a crank with big rod angles in the K engine block.
Financially, of course, it's nonsense : if reboring and coating cylinders is almost affordable, casting and machining a new crank costs more than a new K75 did back in the days.