Yep. Pretty much is that "simple"
DC, the cases don't need any work. 916 pistons are the same diameter as the ones Sigma fitted in your motor. They're heavier though hence the bottom end'd need balancing, and you'd more than likely inherit the 916's rev limit.
Nat..........you've got all the ingredients to do the job, basically you'd just need the balancing done on the bottom end and the barrels machining to compensate for the differing piston heights. Ducati Kamna are in Germany and have the info/parts on their site. Look like they know the game. Maybe worth a call??