Go ahead and try it. If you don't like how it flys with the 4-stroke you can always change.

I have a measly OS 40LA in my Eagle II. It balances perfectly without having to add any weight and has plenty of power, even at my club's bumpy grass field on a hot August afternoon. The Eagle II is a light airframe. Mine is a bit of a kite... wants to float on landings and doesn't handle wind very well. It actually might be better with a heavier engine and some weight in the tail.

If you decide you don't like the 4-stroke you can buy yourself a bushed Thunder Tiger 40 size engine for $50 and save the 4-stroke for your next plane. By the way, you may be able to buy a used case on eBay for the 4-stroke. It's usually something else that goes bad on an engine, so you should be able to find good cases around.