I think the 40 will be plenty of power for normal flying and standard aerobatics (this plane is superb at slow rolls and knife edge flight).

I flew my Rascal 40 for several months with a Thunder Tiger GP42, which is a similar sleeve bearing / air bleed carburetor engine. Even here at 5000 feet it climbed out with authority. Unfortunately, I was never able to tune in a reliable idle with this engine in the inverted postion and the high tank centerline.

I lost patience with the idle problem (and I wanted a bit more power for float flying) so I stuffed in a Saito 72 four stroke a couple of weeks ago. That seems to be a wonderful engine for this airplane, but it is certainly not a budget minded choice.

Greg