Well I might have overstated how 'easy' it is for the average Joe to do with some 20 year old Gov restricted 30mph 2hp moped.
The manufacturer could do it pretty easy (if they wanted to).
First ... why do you want to get 10hp out of an old 50cc ped?
You'd be better off getting a $1000 motorcycle if you just want to go faster.
But ... If you went and bought the motor out of a new KTM50 Senior .. you will have 10 reliable HP with a single speed and centrifugal clutch.
Or you could try to adapt the - airbox .. carb .. cylinder .. piston .. head .. exhaust pipe .. off that same motor to your lower end .. (but it might blow because it wasn't made for 10HP .. but your lower end will blow if you put Nitrous on it too eh ?)
You want to make your own motor make the same HP ?...
2 stroke development is a SCIENCE duck... you could go to engineering school and study hard for 4 years and get a good start ... there are also 2 stroke development textbooks you can read.
You aren't going to get it in one post on a message board.
Its a lot more complicated than that.
... and it requires some math ability.
.. as an example here is one webpage devoted to helping you design your own exhaust pipe