I wouldn't be going with Twin crank cases... I'd be building a horizontally opposed engine with two cylinders sharing a crankcase. This has the advantage of minimal vibration, extremely high crankcase pressures, and of course, twice the displacement.
As for the AV cases modified for Puch stud spacing, I can do those. Treats has the needle bearing, all that would need to be done is make up a bracket. I'll look into it.