I have a Frigioli 604 big bore kit that was fitted along with TTS flowed head and Keihin flat slide carbs to 72degrees race hillclimb Nordwest, you can find write ups about it in the old GeN magazine. these are now not needed as the project went "tits up"
I did not get the GFR rolling chassis I bought, another story posted on here, so the big bore kit was not used by me, my Saturno is fitted with a 558 Nordie engine on the same keihin flat slides, that seems to be enough I did try the flat slides on my Nordie and it did perk it up quite a bit
Ian Calvert used to have a Saturno with the factory kit that took it out to 558. It went pretty well still on the standard carb as far as I know. Andy Wallington had another Saturno with Nordwest engine. The limiting factor is that a standard Saturno frame can't accommodate a twin inlet port head.
Gary Rowe raced a Nordwest on the Isle of Man and later a Spondon with Nordwest engine. That had big bore and 'race' cams. He had great problems with the head joint - as did I with the FPM 604 kit. Mine was giving about 55 bhp on a dyno. The factory Piuma racer 558 was reputably able to give as much as 70BHP, but not for long. The crankcase cracks between the main and balancer shaft bearing. Mick Noblett still managed to bring one home in the singles TT race and put in a 100 mph lap.
"Do not adjust your mind, there is a fault in the reality" 1957 175 Sport, 1993 GFR