Is the engine inverted or sideways? Normally inverted there's no problems mounting a larger engine. Mounting sideways though, even the recommended size engine sticks out the side. Alot of gasoline mounted engines and planes are designed for inverted mounting. A good way to find out is to get the plane first and measure how much room is in the cowl. All engine specs give you pretty detalied mounting dimensions. Then you can decide which engine will fit the best.