I've never been a fan of the 4 stroke motor bicycles, mainly because they have to use centrifugal clutches. The 2 stroke china girl kits have nicer motorcycle style clutches built into the engine. Speed really isn't an issue. With the right gearing and a little work people get the 2 stroke, or 4 strokes up to 40 mph without much trouble. Their safety just depends on your skill as a builder, and how much money you're prepared to spend.
The biggest limitation for me is, you can't ride one on a city bike trail under power, without drawing the ire of everyone that sees you.
In comparison, I can ride my electric at sensible speeds on any trail, and because it's silent no one even looks at me twice.
