I've honestly no idea why so many reference, even buy these engines based on CCs - if ever "there's no replacement for displacement" was a misnomer, it would be in reference to motorizing a bicycle lol
There's 50s pushing 12hp, there's 200s with a mere 5hp - there' no
correlation between displacement and HP in our world, particularly as both are so oft suspect in any case...
...regardless, to answer what I think was the OP's intent - think me an ol' fuddy duddy if you must but for a street bike & even legality aside, I really can't fathom needing much more than an actual 5hp (I run about 1/2 that w/my most powerful build & a mere 1.3hp w/my daily driver) as there's an additional, even critical aspects so often ignored - which would be the machine's overall power to weight ratio & the engines preformance envelope, ie it's usable RPM range & torque as well.
These are the snarky lil details that'll getcha lol, dunno how many times I whipped a "manly" CR500 w/my lil "wuss bike" XR250, to be whipped in turn by some sassy 80 despite the massive power difference - just & only because the weight differences.
Yet, I do understand there will always
be someone that says, "Ya outa stuff a 350 V8 up in it man" no matter what "it" may be