Not to rain on your parade here. But this isn't a problem...most Sachs bikes imported to the USA are 505 1/a engines....dead reliable, but horrible with speed, acceleration, hills etc...
The exception is the 505 1/d, which had a different crankshaft with a longer stroke, and a better cylinder. One of the best stock bikes ever and can hit 40 reliably with a little work.
Sadly due to the clutch design, Sachs bikes can't be kitted or modded for speed as easily as other makes...they just can't hold the power.
Your best bet is gonna be some porting, carb and exhaust work. But really, ur never gonna out perform a 1/D with your engine without tons of work and dedication...
Unless you have a 1/D, in which case imma dummy for posting this...