Since it is advertised as a companion model to the "Perky" I would guess that as a "speed" model, it should have a .15 (2.5cc).

For a trainer or sport model, something less. The one on the Black Hawk Models site in military colors I think has a Gilbert .11 in it. Looks sharp with that setup!

Since those solid wings will increase wing loading, you probably want a light engine.

That is an old timey, nice looking plane.

George