Is there's upgrades that can be done to the Sachs engine?
Not really my ideal drive train to start with but you have options. The bad part of Sachs motors are the weak clutchs. 50-60mph is not really obtainable if your also looking for some sort of reliability out of that motor.
Whats funny is you want to make that bike a 2 wheel bike again... most of us want to take the rear end off it and put it on another moped to make a 3 wheeler. The value of that bike is the rear end, without it the thing is nothing more than scrap metal. Its just in too poor and missing 2 much to fix.