I saw one of those seats on ebay Italy last week the seat. Seats from the 500 Quattro (and the Guzzi 250 TS I think - maybe I'm wrong) will fit. There is a very nice aftermarket single seat that has a rounded hump and a compartment underneath just about big enough for a sandwich, I think it was part of a Tomasselli cafe racer kit. I've just tried fitting the 654 seat to the 750. My Mk1 750 has 190mm between the front edge of the seat hinges just like the 654 but the hinges on the 654 are about 1cm further back so the 654 seat hits the 750's tank. If this wasn't a problem and I could lay the 654 seat down I think would still hit the 750's mudguard at the back.
Let me know if you want photos.