When you click on an object, a small window opens, when you click on that, you get the object's properties. In the motion tab, under orbital elements, is eccentricity. The eccentricity of periodic orbits range from 0 to 1, with 0 being circular and 1 being so impossibly elliptical that the orbit would just be a straight line. Of the numbered Halley-type comets, they have ellipticies ranging from 0.835 to 0.975 with the average being 0.935. try some similar numbers in your sim to see what you like. I hope this gets you started.