I would guesstimate something in the 60cc range should be adequate. Check out the ZDZ or 3W. There are other manufactures such as Taurus,FPE,Zenoah but I am not familiar enough with their engines to recommend anything. I have an 80cc installed in a 26lb aerobatic plane, this motor is more than enough for unlimited vertical. A 60cc with a 3 blade prop should fly your plane with authority.
I would also take a look at
www.rcwarbirds.com they have a wealth of info on warbirds.