there's a lot more to speed than just horsepower.
no one mentioned gear ratio and wheel size.
I can probably make a stock 66 c.c. 2 stroke go 70 mph given the right gear ratio and wheel size.
there's a difference between something being fast, and something being quick.
quickness is what you want on a m.b....get up to top speed as soon as possible.
anything can have a high top speed as long as you have the room for it to actually get there.
it's like drag cars and salt flat cars.
drag cars get from point a to point b (a 1/4 mile) in the quickest amount of time as possible all due to horsepower and gear ratio.(low gearing, tons of horsepower). but after that 1/4 mile run the engine is wound out to the max.
a salt flats car is more focused on top speed, (higher gearing) so it can take a mile or more for it to get up to that top speed.
you can make a 200 horsepower car go 100 mph in a 1/4 mile, or you can make that same 200 horsepower car go 200 mph on the salt flats, all with a simple gear ratio change.
tire size also has a big roll (no pun intended) in this as well.
a taller tire will give you more top speed, and a shorter tire will give you more low end torque.
so...lets say your bike is geared with a 44 tooth sprocket and it has a 24" tall tire on it, and you are getting 25 mph top speed for example.
throw a 26" tall tire on it and you will gain some top end speed.
now, take the same bike with a 24" tall tire and a 44 tooth sprocket, and drop the rear sprocket down to a 36 tooth. now you will gain top speed but loose low end torque....so it'll take longer for it to get up to speed. now, throw a 26" tall tire on it and you will gain even more top speed, but you will loose even more bottom end torque.
gear ratios and tire sizes are the easiest way to make a stock 2 stroke bike engine quick, or fast.
of corse, horsepower does come into play here as well because you can alter the low end torque or the top speed by making more horsepower....everything goes hand in hand when it comes to speed and torque (quickness).
so maybe this guy has his bike geared really high and it takes him 2-3 miles to get up to 90 kmH top speed....it is possible with the right gear ratio.
would i do 90 kph on a bicycle? no way.