Skip right to a light weight 50cc. The 40cc will be good but not as good as 50cc. The additional weight of 50cc is nothing.
ORIGINAL: Redrata
Ok, here I am again, after much more search.
I think it is a difficult question since seems that everybody has powered this plane with a Moki 180.
I think I will go definitely with a Brison 2,4. I will post the results later if someone are curious about performance. I think it is almost a perfect combo. The engine is only .75 lbs over what I have already, so my flying weight with a gas engine will be almost 13lbs, which is great for this model.