The 750 Sei originally came with 530 chain and sprockets, and as far as I'm aware, an o-ring chain will fit without any hassle. At least I've not heard of any problems with installing o-ring chains on them.
The 900 Sei is a different thing entirely. It originally had a duplex final drive chain. I converted my 900 Sei to 530 o-ring chain from the original ½" pitch duplex. Had to put spacers behind the sprockets to move the chain outwards a bit to clear the clutch pushrod seal. I wrote up a technical article on it at the time for the Benelli Club magazine. I can provide a PDF of it if anyone is interested in doing the same conversion.