A quick rule of thumb is to use the same displacement 2-stroke gas engine as the 4-stroke glow engine it replaces. Example, my Enya 120R-4C turns a 16x6 MS prop at 8,800 rpm. My RCGF Stinger 20RE turns a 16x7 Power Prop at 8820 rpm. Both engines have the same 1.2 cubic inch (20cc ) displacement. The gasser is slightly more powerful, but not by all that much. Replacing the Saito 1.8 cubic inch glow four stroke engine with an equal power gas 2-stroke engine would require a 26 to 30 cc gas engine.