The larger displacement engines have a tad more bottom end torque but for the most part there isn't a great deal of difference in the engines. At your weight you might want to go with the bigger one but there are lots of 49s carrying as much or more weight than you. By the way here's a link you might find interesting. http://motorbicycling.com/f3/blueprints-occ-stingray-motor-mount-10210.htmlso whats the most common motor used. i weigh in the 200 lb range and dont want to be under powered.
see a lot of 48-49 cc but would the 66/80 be to much?