This last year I reverted my '85 spree back to a more "original" configuration due to the gas milage.
I bought the moped in 86 with 300 miles on it. After 2000 miles I blew a hole ( water corrosion) in my exhast pipe. At that point this is what I did:
Placed a modified Honda 80 exhaust pipe onto it to reduce the restricted flow.
Placed a circular airfilter on the carb to get rid of the airflow restriction.
Used a Bosch Super Platinum Spark Plug - if you do anything do this! They run better at high RPM's then even any Japanese manufactured plugs and will give consistant ignition.
Ran Evenrude high performance 2 cycle oil.
Ran super unleaded gas - with the occasional 108 octane boost.
I was going to port the head and give more top end, but at this rate I was already passing vehicles on the local freeway doing 55+
Anything more start looking for a carburator from an 80 or 90 to put into it.
The airflow restriction is in dire need to be removed.
Might want to find a larger rear tire in the 3.5 range to fit on the back side to help stabilize it. No guarantess, but that was also my next move - if I remember right.
I still run the Bosch Plug. But everything else is gone back to the original and sold .
Good luck - but I will leave you with this parting words:
The faster you go the more squirlier it will get.