General rule of thumb: Don't buy a used engine! very bad!
Trin/Picco: Tons of power, somewhat hard to tune, not ROAR legal for racing (.15) You also need a rear exhaust manifold.
O.S. CV-X: Excellent motor, but it's used here, so stay away. You can pick one up for about $119 w/ a pullstart at Tower. It practically tunes itself and has great power.
M-16: Never used one, but they look very good. If I had to pick one here, this would be it. (unless, you feel you're a good tuner, in that case, I'd tell you to get the Trin/Picco .15RE) They're also darn cheap brand new, too.