1.1hp/cubic inch (375ish) is very easy using modern machining, and a modern cam grind, with all other parts being factory (carb, intake, ignition, etc) including exh manifolding. 1.25hp/cu inch (425) will need better intake, more compression, better ignition and bigger carb, maybe gasket match the intake ports. Over that, you need to do soemthing with the heads in terms of proting or replacement. The biggest gains are from the beter machining (which blueprints the build and uses modern theory in terms of valve jobs) and the modern cam grind.