To quote Wiffleball from this thread:
We actually spend a great deal of time and effort trying to get speed from moped engines. The engines themselves are a point of pride. Not because they are great engines, but because there is a challenge in it. There is a real art in trying to squeeze power out of an engine that was purposely built to go slow.
Putting a 125cc engine on a moped is about the equivalent of riding a bicycle in the Boston Marathon.
What you end up with is a slow motorcycle with bad brakes, poor suspension, and a weak frame, not a fast moped. It’s especially pointless to go through the work when Honda CT90, CT110, and C70 frames are everywhere for next to nothing.
Lastly, Lifan builds a decent engine, but it’s over priced compared to a used Honda. You can snag a REAL HONDA engine and rebuild it for less.
One example why it is worth it. Shown by example, built by Filipp, you be the judge.