A 140 glow engine is 1.4 cu inches. glow engines are typically measured in cubic inches whilst gas engines are typically measured in cubic centimeters. The conversion factor is about 16, so 1.4 cubic inches equals 22 cubic centimeters. So you need a gas engine in the range of 22 cc's. Perhaps a little bigger since gas engines do not produce as much power as glow (for the same displacement). There are engines in the 22 - 35 cc range. Overall your plane will be a little heavier with gas because they are a little heavier. Gas engines have to have a way to make a spark and this adds weight. a glow engine can use a simple glow plug - very light.