erect11, I have the same plane. Either engine will work but the extra weight of the 31cc will not hurt. I'm using a 25cc McCoughal conversion with electronic ignition. Even though it is heavy I still had to mount both ign. and radio batteries under the cowl and still needed to add some weight to the nose. Total weight of the plane is about 16 lbs. I rebuilt the gear mounts and added flaps. Plane flys great. I'm using Wingspan electric retracts. Here is the link http://www.wingspanretracts.com/LGM685. I had air retracts which were not working good and replaced with these. I have 15 flights on them and so far they have worked great. Using Robart struts 5/16" dia. and 4" wheels.