Mounting O.S. 91 Surpass
I have used the OS .91 Surpass for several years. On all the planes that show a two stroke engine I move the firewall back an inch to inch and a half. The plans of my Bingo show two different nose sections one for a four stroke ( the plans are in FL so I can't tell you the measurement).
Moving the engine back also helps with the balance as four strokes are a bit heavier.