The plane manufacturer suggests a .60 size two stroke. In my opinion, anything less than a .90 four stroke would be underpowered. The plane weighs 7.5 lbs.

[link=http://airborne-models.com/html/productdetails.asp?ProductID=30]TWM Zero .60[/link]

I would do a search of the plane on this site and see what other people are using, just to be sure a .90 four stroke would fit.