To determine is the compression is bad ( worn out ring or sleeve), first you need to prove that the valves are not sticking or leaking. The easiest way to determine this is to turn the engine over by hand while listening to the exhaust and intake port for a hissing sound. If you do not hear a leak there, then put a small amount of oil into the glow plug hole, reinsert the glow plug, turn the engine over to see if the compression has increased. (open the carb all the way by the way. If you have it closed the compression can seem low because not enough air is being sucked in and the compression can seem low). If the compression does increase with a little oil then you might want to tear the engine down but I would try to get it started again as I have had the experience of having thought the compression was bad ( low restistance when cranking by hand and then after running the engine again everything would be ok). If you do have ring trouble and want to resleeve and put in a new ring the the piston will come out by pulling the wirst pin out through the
hole in the back of the clyinder wall after the sleeve is lifted enough. Sometime the pin is stuck because of congealed oil and it helps to heat it first and the try to pull it out by inserting a small wire with a slight tang on it. You have to sort of wedge it sideways and pull. Some pins are harder than others. Don't lose the teflon scuff pads. It helps to not pull the sleeve all the way out. That way the piston is not floating sideways. Hope this helps a little. I ran one of these engines for more than 20 years. I re- sleeved it and ringed it twice.