I learned to fly on an eagle 2 as well with an o.s. fp 40. I would say that engine is a little weak for flying off of a grass field. A little bigger engine would be better for that.

I have not run any 4 stroke engines, but I wouldnt be afraid to try the .65 4 stroke in an eagle2. However, as others have mentioned, prop to ground clearance could be an issue. Also, as was mentioned, the eagle2 is a floater, so you need to get down to a very slow idle to land. Try a prop with a lower pitch to keep the speed down. Say a 4 or 5 inch pitch would probably work well. However, going down to a lower pitch usually means increasing the prop length, causing more clearance issues.

A good engine for this plane would probably be a .46 la. I used a 46 fx on mine for a while, and it had plenty of power then. Grass take offs were no problem then.