That is light construction, but the plane is made to fly. Personally, I couldn't build one that light from scratch.

The 40 4-stroke would fly it around more like a fat wing trainer or a sport plane. A .56 would be better. For 3D type flying, you'd need something in the .70-.75 range. I had a Saito .72 and an OS .70 available, but didn't want to take the trouble to re-route the thrlttle cable. Besides, the wife got me the OS .55 along with the Tango so I was obligated to ues it. The .70 is now in a Stinger 40 ARF. $80 on sale from Tower. I didn't need it, but it was too cheap to pass up.

One problem with the .40 FS, or even a Saito .56, is nose weight. You might need the battery way up front under, or over, the tank. With a small engine, I definitely hate to add dead weight. I'm using metal gear, digital servos in the rear, but for a light engine, I think I would go with nylon gear standards. You ought to be able to save an ounce or so right there. This would probably translate to 4 ounces in front.

The old OS .40 never was a power house. I figure it would be like a .25 on the nose. I did solo a guy flying one on a 3-channel Sig Kadet Senior. It was slow, but it putt-putted around nicely. It was an old style trainer where you could release the sticks and the plane would automatically return to level flight. I saw him do it on final once when he got messed up in a right turn. I was about to snatch the transmitter when he released and bingo, the plane leveled out. He had turned left, then corrected wrong. He turned on around and went on and landed.

I'll bet one of the older Saito .65s would be nice and not overly powerful.