remove the plug, attach the plug boot, clamp the metal part of the plug to the engine block fins and crank it over. You should see a spark on the plug.
if you've got spark and it's no-go with Starter Fluid, compression is the last thing.. open the engine up and look.
You said everything is fine now.. theres spark.. the carb is fine... the piston seems to be ok. Who am i to argue? You've got the bike in front of you.. all i know is what you type. :)