There are 16 and a fraction cc's in a cubic inch but there isn't a direct relation from one type of engine to another. Most of the newer kits and arfs will give the engine range in glow and gas. Look at the ads in the magazines. There are some smaller engines on the market that would work in 1.20 to 1.50 sized airplanes. People have even put gassers on the Funtana 90 that normally uses a glow 91 FS. The problem you run into is that a plane designed for glow may require cutting the cowl all to pieces to fit in a gasser and its muffler. To go Gas or Glow can also depend on the airframe and the firewall. May have to add structure which adds weight. There is no simple answer. A plane with a tapered cowl may hide a glow engine but a gasser is halfway out the bottom.