No offense, but 120hp on a gasoline, ride-able cb750 without forced induction is pretty much an impossibility.
Mike Riecks personal 900cc cb750 tops out at just about 100hp right now. I dunno if there are any ride-able normally aspirated 750's that make more, or even the same, but if there are, they are few and far between.
One way to get closer to that is to take a look at Mikes cnc machined 1000cc jugs. There's a sticky up there in this section of the forum.
In any case, set your sights on 90rwhp. It's doable, wont cost you tens of thousands of dollars, and all the things required to do this have been documented.
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