I have some old gliders that were builds of my Dads. Some of them have small cox engines mounted on the top of the wing just for getting the plane up to altitude I assume. It runs wide open like a control line or free-flight model until the small tank runs out of fuel & by then you have gained enough altitude to sail around.

The OS .40LA is much too big for this type of application imo. From looking at the plane in your pic, your plane is no bigger than the ones Dad built & the engine on these is only like a cox baby bee or an .049 I would have to look at one to be sure. If my math is correct your listing a total weight of 900 grams between wing & fuse wich comes out to about 1 lb. It shouldn't take near that much engine to get a plane that light up in the air. An .049 to .074 should be plenty I would think. Plus they will have the right type of small fuel tank mounted on the back of the engine. You don't want it to run for very long. Just long enough for you to get enough altitude for gliding right? An engine on a glider is just a method of launching it.

http://www.mh-aerotools.de/airfoils/cox_frameset.htm


Take a look at these engines. I believe this is more along the line of what you would need for a light glider. I would post this question in the gliders & sailplanes forum though. Those guys will know better than I do about this because I don't fly gliders. I'm just going by what I see on the gliders I have. Those guys are into gliders though so they will know what you need.