I'd try west systems e-poxy its a 1 to 1 mix and they have a few different fillers aka micro balloons that I would mix in if it were me.
how about four o.s. .20 four strokes? you could try http://mecoa.com/ for engines.
Now lets see if I have this right, you placed new spars in the areas where someonbe had removed them and replaced the center rib as well?
If so then you did a fine job but also you will want to double the spars up in the area where the old meet the new.