The 750 crank can be used in the crankcases but not a drop in replacemnt for a 900 crank.
The stroke is 7mm longer on 900. The cylinder and studs are longer. The cam chain is different length due to the stroke length. The primary chain is different length. The tensionner and guide are different. The front cam chain guide is different in 900 vs 750.
The 750 was the bastard child of this bunch. When Honda took the 750 to 900 it was .100" bore but .280" more stroke and all the other parts changed as well to make the room for the longer stroke.
Even super crude assembly,the piston will be .280" short and the primary chain,primary chain tensionner and top guide need to be changed.