Well... the plane will do OK with a 1.20 FS ( 4 stroke ).

But it will have MUCH more power with a 1.20 AX ( 2 stroke ).

However on this Biplane I prefer a four stroke.

So to get the equivalent power as the 1.20 AX, on a 4 stroke engine you would have to move up to around 1.60 CU to 1.80 CU four stroke engine.

A comparison between the 1.20AX and a larger four stroke engine would be more equitable.

As it is the 1.20AX has more power. If I had to choose between just those two, I'd go with the 1.20 AX, but I would be hit with higher fuel consumption.