The problem with casting is you have to make an oversize core to compensate for the shrinkage, by time you do that and the subsequent machining operations castings need you would be better off machining the crank case from bar stock, welding on any bosses that you would need for mounts etc.
Then you can prove out your design and make adjustments before spending all the money on cores etc.

Plus not have to deal with casting porosities inclusions etc you would have to use vacuum-assisted casting to avoid these isssues.