allo Nelly,
"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."
Perhaps I am getting confused with 955 pistons and the size etc.
What about the idea of fitting some hotter cams from the 748R or SP. Also, I have always fancied putting 916 injectors on if they would supply more go juice!!
DC.