Wow!... that is high altitude!
Normally I would say to go with this plan.
GAS and OIL
With modern 2-stroke oils you should probably pre-mix between 3 oz.(43 to 1) or 4 oz.(32 to 1) of oil per gallon of gas.....and mopeds are designed to run on the lowest octane of gas you can buy (87 oct.) because they are afraid that somebody who doesn't know better will put the cheap stuff in..... and higher octane will not make it run better or faster or make more power.... Make sure you buy 2-stroke oil.
I have not found it necessary to go with synthetic oil.
Lead additives serve no purpose in 2 stroke.
But at your altitude you might gain a little performance by milling the head for more compression.
And you get more top speed with a tuned pipe like a Bi-Turbo... but I don't know if they make one specifically for your ped.
Either way if you start making changes watch the spark plug color...if it is getting white you are in danger of a seizure.