Hi Paul,
The size I would like will be smaller, around 40cc to 70cc (using two 20cc to 35cc engine). I also think the crank case is the place to start, but it's also the more complicate part to design. In a two stroke the mixture pass in the crank case, so the design is important and it's that knowledge I don't have.
I will begin to study how the actual crank case is made, and see if I could use the actual unit.
I will use SolidWorks for the cad software.
CrazyRonny