Why not?

I can build reinforcement into the firewalls and fuse sides if needed.

A 120 4-stroke glow engine weighs just a bit over 2 pounds. Scotts 25cc Toro weighs only 3.5 pounds. I can't see why a 20 or 21cc engine wouldn't be a bit lighter than that. Maybe 3 pounds if I could find a nice, light engine.

You saying an airframe with 87" of wing won't support an additional 2 pounds worth of engines? I could use little 8 or 10oz fuel tank. I'll bet the overall weight gain over 120 4-strokes would come out to about 1 or 1.5 pounds. That airplane can handle that much--no problem.

Anyone know of any small gassers--not too heavy--that I could put on this plane?