Well for smoothness my 48 cc engine is pretty darn good, and it pulls 6 percent grades with my 155# weight at 20 mph, all day if needed. But it's really detuned to run better at midrange and have a broad torque curve.
What makes these engines really vibrate is that they don't supply them with an output sprocket that is concentric! Nearly all of them have some out of roundness, mine was especially bad and I ended up grinding it lower on the roller lands parts of the sprocket's high spot, filling in the low spots with hard stainless TIG weld filler, and then carefully hand filed the concentricity back into the sprocket.
That's a lot of trouble to go through, but it's eliminated what was the worst vibration my engine produced, the lower frequency, output shaft sprocket eccentricity judder. You can buy new sprockets.. I've found that out of 4 that I bought, none were much better than the stock one.