i don't believe it matters how much a person weighs, unless it's so much that the bike can't support the rider's weight. or they're so big that wind resistance comes into play.
weight will affect acceleration, but given a long enough run, a 250lb person and a 135lb person will get the same top speed, if all other factors are the same.
i think no matter what you add onto one of these motors, the limitations still lie within the motor itself. a good exhaust and carb will help it a lot, but it's the same old story of the "our new bolt on muffler will give you a 15HP boost" B.S. you see with aftermarket car parts.
i'm not saying you shouldn't get that expansion chamber or that race carb, i'm just saying you should be realistic about it. you're starting with a less than 3HP motor. adding a pipe ain't gonna get you 5HP. just ask a racecar builder how hard it is to get those few extra horses...
it's too bad no one is doing any dyno testing on all the available products so we can see what we're actually getting.
unless the HP can be significantly raised, gearing is the only way to get a higher top speed.
this is how i see it.
you start with an engine that claims 2.5 HP.
port, polish, deck, and clean that thing out till it actually has 2.5HP.
then add the expansion chambers and carbs to help it use that 2.5HP. (and if you're lucky, you might squeeze a tiny bit more out of it.)
the key here, is making it all work nice and smooth together.
then gear it right for you. if you wanna go fast, go with the smallest sprocket that'll still pull you.
if you wanna cruise, get something midrange, so it won't rev-out, and it'll still pull you up those hills.
if you wanna do wheelies and beat cars off the line, get a big sprocket.
none of what i typed is scientifically proven as fact, but it works like this in my experiences.
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