the featherlite 20cc could work, but on a 40 size plane it really needs to be on electric ignition (or gas/glow, if you look through the thread there are options in what you can use) to work. Like I said in your other post, those featherlites are super light, but are also dogs unless you modify the exhaust port duration (they come at around 110 degrees when it should be 150 degrees for better power). With that plane and a magneto engine you would have to chop the nose of the plane (I wasn't talking about the engine before) down to just in front of the wing, with electric ignition you can move the ignition module and battery back behind the engine where the fuel tank normally goes to get the COG back further. And for a battery, it doesn't have to be complex, I've taken cordless phone batteries apart and made myself a receiver pack, a 800mah battery should last around 3 flights before a charge is needed. Trimming down aluminum from the engine doesn't add up fast at all, you could work for a few hours and only get a couple ounces, the weight is in the crankshaft...

good luck, I'm actually trying to put a 18cc featherlite on a 40 size stick, need to get my bandsaw going before I get any further on that.