I was just reviewing some old posts and ran across yours. I'm late to respond I know. I recently bought a used Enya .19 on eBay that looked like new in the photos. When it arrived I found the piston rusted fast to the sleeve and the sleeve rusted and expanded inside the aluminum crankcase. I simply tore down the engine to where i could get at the rusted stuff and heated the cylinder assembly with a propane torch then let it cool hoping I had cracked the rust loose a-bit. I reheat again and the cylinder sleeve and piston began to move as a unit. I kept heating and moving the cylinder liner and it finally slipped out of the crankcase. I then applied heat to the liner and piston assembly and tapped on the piston crown with a small hammer and a short piece of 3/8" hardwood dowel. The piston broke loose. I cleaned up the piston and the outside of the cylinder sleeve with a light wire wheel in my Dremel moto tool. I didn't want to use any abrasives. I then cut a slot in the end of a piece of 1/4" wooden dowel and cut a 1/2" wide strip of 600 wet/dry paper. I folded the strip over on itself with the abrasive sides exposed and slipped it into the slot in the dowel. I then chucked the dowel into my elctric drill motor and lightly honed the cylinder with light machine oil, WD40 should work. I wanted the bore smooth but not down to bare metal. The engine is now assembled and has such good compresion that you almost can't turn the engine over by the prop hub.

I was lucky but it might work for you also.

Bill