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